Ultrastructural study of the male gamete of Glossobothrium sp. (Cestoda: Bothriocephalidea: Triaenophoridae) a parasite of Schedophilus velaini (Perciformes: Centrolophidae) in Senegal

This paper describes the ultrastructure of the male gamete of Glossobothrium sp. (Bothriocephalidea: Triaenophoridae). The mature spermatozoon of Glossobothrium sp. is filiform and possesses two axonemes, a single helicoidal crested body, a parallel nucleus, parallel cortical microtubules and granules of glycogen. In Glossobothrium sp. we describe for first time a 200-250 nm thick crest-like body in the Bothriocephalidean. The anterior part of the spermatozoon exhibits a ring of 27 electron-dense cortical microtubules encircling the first axoneme. This structure persists until the appearance of the second axoneme. When the ring of electron-dense cortical microtubules disappears, the spermatozoon exhibits two bundles of thin cortical microtubules. The posterior part of the spermatozoon contains the posterior extremity of the second axoneme, the posterior extremity of the nucleus and few cortical microtubules. Soon nucleus disappears and the axoneme is disorganized. Thus the posterior extremity of the spermatozoon of Glossobothrium sp. exhibits only singlets produced by the disorganization of the doublets of the second axoneme and few cortical microtubules. This type of posterior extremity of the mature spermatozoon has never been described previously in the Triaenophoridae.     

Ultrastructural study of spermiogenesis and the spermatozoon of the proteocephalidean cestode Barsonella lafoni de Chambrier et al., 2009, a parasite of the catfish Clarias gariepinus (Burchell, 1822) (Siluriformes, Clariidae)

Spermiogenesis in the proteocephalidean cestode Barsonella lafoni de Chambrier et al., 2009 shows typical characteristics of the type I spermiogenesis. These include the formation of distal cytoplasmic protrusions forming the differentiation zones, lined by cortical microtubules and containing two centrioles. An electron-dense material is present in the apical region of the differentiation zone during the early stages of spermiogenesis. Each centriole is associated to a striated rootlet, being separated by an intercentriolar body. Two free and unequal flagella originate from the centrioles and develop on the lateral sides of the differentiation zone. A median cytoplasmic process is formed between the flagella. Later these flagella rotate, become parallel to the median cytoplasmic process and finally fuse proximodistally with the latter. It is interesting to note that both flagellar growth and rotation are asynchronous. Later, the nucleus enlarges and penetrates into the spermatid body. Finally, the ring of arching membranes is strangled and the young spermatozoon is detached from the residual cytoplasm.The mature spermatozoon presents two axonemes of the 9 + ‘1’ trepaxonematan pattern, crested body, parallel nucleus and cortical microtubules, and glycogen granules. Thus, it corresponds to the type II spermatozoon, described in almost all Proteocephalidea. The anterior extremity of the gamete is characterized by the presence of an apical cone surrounded by the lateral projections of the crested body. An arc formed by some thick and parallel cortical microtubules appears at the level of the centriole. They surround the centriole and later the first axoneme. This arc of electron-dense microtubules disorganizes when the second axoneme appears, and then two parallel rows of thin cortical microtubules are observed. The posterior extremity of the male gamete exhibits some cortical microtubules. This type of posterior extremity has never been described in proteocephalidean cestodes. The ultrastructural features of the spermatozoon/spermiogenesis of the Proteocephalidea species are analyzed and compared.     

Sperm characters of the digenean Nephrotrema truncatum (Troglotrematidae): a kidney parasite of Crocidura russula (Soricidae) and their phylogenetic significance

ABSTRACT

Spermatological characteristics of the troglotrematid digenean Nephrotrema truncatum, a parasite of the shrew Crocidura russula, have been investigated by means of transmission electron microscopy. The ultrastructural study reveals that the mature spermatozoon of N. truncatum exhibits many ultrastructural characters previously described in most gorgoderoideans. These are two axonemes of the 9+‘1‘ trepaxonematan pattern, four attachment zones, a lateral expansion, an external ornamentation of the plasma membrane associated with spine-like bodies and cortical microtubules, and in the posterior part of the anterior spermatozoon region, two bundles of parallel cortical microtubules with the maximum number located in the anterior part of the spermatozoon, a nucleus, two mitochondria, and granules of glycogen. The obtained results are compared with those of other digeneans, particularly the Gorgoderoidea. The sperm cells gorgoderoideans are of type IV, characterised by a 9+‘1‘ pattern of axonemes, the presence of an external ornamentation associated with cortical microtubules and located in the posterior area of the anterior extremity, the presence of two bundles of cortical microtubules, the maximum number of cortical microtubules located in the anterior region of the spermatozoon, and the presence of generally two mitochondria. However, dicrocoeliids and troglotrematids have spermatozoa with ornamentation of the plasma membrane and lateral expansions.     

Ultrastructure of Spermiogenesis and the Spermatozoon of Mathevotaenia herpestis(Cestoda), Intestinal Parasite of Atelerix albiventrisin Senegal

Spermiogenesis in M. herpestisbegins with the formation of a differentiation zone which contains two centrioles associated with an electron–dense, finely granular material. This granular material very quickly becomes striated, a median cytoplasmic extension forms, one of the centrioles becomes laterally oriented in a cytoplasmic bud and the other gives rise to a flagellum. After the migration of the nucleus, a helicoidal crested–like body forms, then the old spermatid separates from the residual cytoplasm. The mature M. herpestisspermatozoon exhibits an apical cone of electron–dense material, a crested–like body and cortical microtubules which are electron–dense centred and spiralized except at their posterior extremity where they are parallel to the spermatozoon axis. The axoneme is of the 9 + ‘1’ pattern. It reaches the posterior extremity of the gamete where the cytoplasm is very electron–dense. The presence of centrioles flanked by ‘striated roots’ has never, to our knowledge, been reported in a platyhelminth. Likewise, a nucleus with an annular cross–section and unevenly distributed electron–dense peri–axonemal material has never been described in a cestod.     

Spermiogenesis and spermatozoon ultrastructure of the davaineid cestode Raillietina micracantha (Fuhrmann, 1909)

Miquel, J., Torres, J., Foronda, P. and Feliu, C. 2010. Spermiogenesis and spermatozoon ultrastructure of the davaineid cestode Raillietina micracantha. — Acta Zoologica (Stockholm) 91 : 212–221 The spermiogenesis and the ultrastructural organization of the spermatozoon of the davaineid cestode Raillietina micracantha are described by means of transmission electron microscopy. Spermiogenesis begins with the formation of a zone of differentiation containing two centrioles. One of the centrioles develops a free flagellum that later fuses with a cytoplasmic extension. The nucleus migrates along the spermatid body after the proximodistal fusion of the flagellum and the cytoplasmic extension. During advanced stages of spermiogenesis a periaxonemal sheath and intracytoplasmic walls appear in the spermatids. Spermiogenesis finishes with the appearance of two helicoidal crested bodies at the base of spermatids and, finally, the narrowing of the ring of arched membranes detaches the fully formed spermatozoon. The mature spermatozoon of R. micracantha is a long and filiform cell, tapered at both ends, which lacks mitochondria. It exhibits two crested bodies of different lengths, one axoneme of the 9 + ‘1’ pattern of trepaxonematan Platyhelminthes, twisted cortical microtubules, a periaxonemal sheath, intracytoplasmic walls, granules of glycogen and a spiralled nucleus. The anterior extremity of the spermatozoon is characterized by the presence of an electron‐dense apical cone and two spiralled crested bodies while the posterior extremity of the male gamete exhibits only the axoneme and an electron‐dense posterior tip.     

Ultrastructure of the spermatozoon of Bothriocotyle sp. (Cestoda: Bothriocephalidea), a parasite of Schedophilus velaini (Sauvage, 1879) (Perciformes: Centrolophidae) in Senegal

The mature spermatozoon of Bothriocotyle sp. is filiform and tapered at both extremities. It possesses 2 axonemes of unequal length, showing the 9 + "1" pattern of Trepaxonemata. The anterior extremity exhibits a crest-like body. Thereafter, the crest-like body disappears, and the first axoneme is surrounded by a ring of cortical microtubules (about 27 units) that persist until the appearance of the second axoneme. This ring of cortical microtubules is characteristic only for species of Bothriocephalidea and represents a very useful phylogenetic character. The spermatozoon cytoplasm is slightly electron-dense and contains numerous electron-dense granules of glycogen in several regions. The anterior and posterior extremities of the spermatozoon lack cortical microtubules. The posterior extremity of the spermatozoon of Bothriocotyle sp. possesses a nucleus and a disorganized axoneme, which also characterizes spermatozoa of the Echinophallidae studied to date.     

Ultrastructure of the spermatozoon of Anomotaenia quelea (Mettrick, 1961) (Cestoda,Cyclophyllidea, Dilepididae), an intestinal parasite of Quelea quelea (Aves,Ploceidae) in Senegal

The mature spermatozoon of Anomotaenia quelea exhibits an apical cone of electron-dense material and two helicoidal crest-like bodies. The apical cone near its base is surrounded by a lucent cytoplasm and a spiraled layer of cortical microtubules. The crest-like bodies are of different lengths, spiraled and make an angle of 30–40° to the hypothetical spermatozoon axis. The axoneme is of the 9 + ‘1’ trepaxonematan pattern and is surrounded by a periaxonemal sheath of electron-dense material. The cytoplasm contains in regions III and IV numerous electron-dense granules situated between the periaxonemal sheath and the cortical microtubules. The posterior extremity of the spermatozoon of A. quelea exhibits a nucleus and a disorganized axoneme and cortical microtubules. This type of posterior extremity of the mature spermatozoon has never been described previously in a Dilepididae. Similarly, two crest-like bodies have not been observed before in a dilepidid cestode.     

Spermiogenesis and spermatozoon ultrastructure of the cranial digenean Troglotrema acutum (Leuckart, 1842)

Ultrastructure of spermiogenesis and the main characters of the mature spermatozoon of Troglotrema acutum are described by means of transmission electron microscopy. Specimens were obtained from the nasolacrimal sinuses of an American mink (Mustela vison). Spermiogenesis in T. acutum follows the general pattern of digeneans. The zone of differentiation is a conical-shaped area bordered by cortical microtubules and delimited at its base by a ring of arched membranes. This area contains 2 centrioles associated with striated rootlets and an intercentriolar body between them. The centrioles develop 2 free flagella that grow ortogonally to the median cytoplasmic process. The posterior flagellar rotation and proximodistal fusion of the free flagella with the median cytoplasmic process originate the spermatozoon. The mature spermatozoon of T. acutum is characterized by the presence of 2 axonemes of different lengths presenting the 9+'1' trepaxonematan pattern, 2 bundles of parallel cortical microtubules, 2 mitochondria, a nucleus, and granules of glycogen. These ultrastructural characters are compared with other digenean species previously studied and the importance of different spermatological features is discussed.     

Spermiogenesis in a Scutariellid (Platyhelminthes)

Spermiogenesis and spermatozoa were studied by transmission and scanning electron microscopy in Troglocaridicolasp., a scutariellid epizoic on a cavernicolous freshwater shrimp. Spermiogenesis involves elongation of the spermatid in which the nucleus elongates, but remains close to the common cytoplasmic mass. Flagella first grow in opposite direction and at a right angle to the cytoplasmic shaft, and centrioles show associate structures. Later, the two centrioles rotate and the flagella emerge parallel, but still perpendicular to the shaft. An apical process elongates at the extremity of the spermatid shaft. The spermatozoon shows active flagellar beating and undulations of the sperm body. The spermatozoon comprises an anterior ‘corkscrew’ region, the flagellar insertion region, a cytoplasmic region and a posterior nuclear region. The corkscrew contains an electron dense structure, not membrane-bound, originating from the apical process of the spermatid. The flagella show the 9+‘1’ pattern, usual in Platyhelminthes. The cytoplasmic and nuclear regions show a cortical row of about 50 twisted longitudinal microtubules surrounding a row of electron dense, and not membrane-bound, 25-nm granules. These granules are original structures and seem to be known only in a few Platyhelminthes species in which a non-flagellar movement of the spermatozoon occurs. Thus, it is hypothesised that the 25-nm granules play a role in cellular motility. Sperm ultrastructure in Troglocaridicolashows major differences to that in the temnocephalids. It is therefore concluded that the phylogenetic position of the scutariellids within the Temnocephalidea should be reinvestigated.     

On sperm ultrastructure,spermiogenesis and the spermatophore of Heterolaophonte minuta (Copepoda,Harpacticoida)

Summary The spermatophore, mature spermatozoon and spermiogenesis of Heterolaophonte minuta have been investigated by light and electron microscopy. The spermatophore contains three different secretions which are responsible for the discharge of the contents of the spermatophore, the formation of the fertilization tube and the storage of the spermatozoa. The spermatozoon represents a type new for the Copepoda. It is a filiform cell about 25 m in length, ellipsoid in transverse section and tapered at the posterior end. The elongated nucleus contains chromatin fibrils and does not possess a nuclear envelope. Posterior to the nucleus, six mitochondria are placed one after the other. The posterior part of the spermatozoon contains parallel pseudomembranes. The gamete is not helically twisted and is without a flagellum and centrioles. The most remarkable feature of the spermatozoon is an osmiophilic cap in front of the nucleus. This cap corresponds to the acrosome of the spermatozoon. Early stages of spermiogenesis take place in the testis, where the spermatids are incorporated into accessory cells. The origin of the chromatin fibrils and the glycocalyx, as well as the breakdown of the nuclear envelope and centrioles, represent the final steps of spermiogenesis which occur in the vas deferens.     

Spermiogenesis and spermatozoon ultrastructure of the dilepidid cestode Molluscotaenia crassiscolex (von Linstow, 1890), an intestinal parasite of the common shrew Sorex araneus

Marigo, A.M., Bâ, C.T. and Miquel, J. 2011. Spermiogenesis and spermatozoon ultrastructure of the dilepidid cestode Molluscotaenia crassiscolex (von Linstow, 1890), an intestinal parasite of the common shrew Sorex araneus. —Acta Zoologica (Stockholm) 92 : 116–125. Spermiogenesis in Molluscotaenia crassiscolex begins with the formation of a differentiation zone containing two centrioles. One of the centrioles develops a flagellum directly into the cytoplasmic extension. The nucleus elongates and later migrates along the spermatid body. During advanced stages of spermiogenesis, a periaxonemal sheath appears in the spermatid. Spermiogenesis finishes with the appearance of a single helicoidal crested body at the base of the spermatid and, finally, the narrowing of the ring of arched membranes causes the detachment of the fully formed spermatozoon. The mature spermatozoon of M. crassiscolex exhibits a partially detached crested body in the anterior region of the spermatozoon, one axoneme, twisted cortical microtubules, a periaxonemal sheath, and a spiralled nucleus. The anterior spermatozoon extremity is characterized by the presence of an electron‐dense apical cone and a single spiralled crested body, which is attached to the sperm cell in the anterior and posterior areas of region I, whereas in the middle area it is partially detached from the cell. This crested body is described for the first time in cestodes. The posterior extremity of the male gamete exhibits only the disorganizing axoneme. Results are discussed and compared particularly with the available ultrastructural data on dilepidids sensu lato.     

Ultrastructural characteristics of the mature spermatozoon of the digenean Sclerodistomum italicum (Stossich, 1893) (Hemiuroidea,Sclerodistomidae) intestinal parasite of Hypocanthus amia (Teleostei,Carangidae)

The mature spermatozoon of Sclerodistomum italicum is filiform, tapered at both ends and shows the following features: 2 axonemes of the 9 + “1” pattern of the Trepaxonemata, mitochondrion, nucleus and parallel cortical microtubules. The specific features of the spermatozoon of S. italicum include the simultaneous presence of two types of extramembranous ornamentations, the presence of short cortical microtubules in the anterior part of the spermatozoon and the presence of only one bundle of cortical microtubules in the median part of the spermatozoon. Thus far these structures are known only in the Hemiuroidea. The presence of filamentous ornamentation in the anterior extremity of the spermatozoon has not previously been described in the Sclerodistomidae. Similar to spermatozoa of other hemiuroideans, S. italicum lack spine-like bodies described in spermatozoa of many digenean taxa. The posterior extremity of the spermatozoon exhibits the same ultrastructural characteristics typical of the Hemiuroidea.     

Spermatological characters in the diphyllobothriidean Schistocephalus solidus (Cestoda)

Levron, C., Yoneva, A. and Kalbe, M. 2011. Spermatological characters in the diphyllobothriidean Schistocephalus solidus (Cestoda). —Acta Zoologica (Stockholm) 00 : 1–8. The spermiogenesis and the mature spermatozoon of Schistocephalus solidus (Cestoda: Diphyllobothriidea) are described using transmission electron microscopy. Spermiogenesis in S. solidus begins with the formation in the spermatid of a differentiation zone surrounded by cortical microtubules and delimited by arching membranes. This conical area presents two centrioles associated with striated rootlets and a median cytoplasmic extension between them. The centrioles are separated by an intercentriolar body composed of three electron‐dense plates dividing four electron‐lucent plates. The centrioles give rise to two flagella that undergo a rotation and later fuse proximodistally with the median cytoplasmic expansion. The presence of an electron‐dense material in the distal part of the differentiation zone is observed in the early stage of spermiogenesis. This pattern corresponds to Type I spermiogenesis according to the classification proposed by Bâ and Marchand (Mémoires du Muséum National d’Histoire Naturelle 1995; 166 : 87). The mature spermatozoon of S. solidus presents the Type I pattern defined by Levron et al. (Biological Reviews 2010; 85 : 523). It consists of five regions that exhibit two axonemes, parallel cortical microtubules, nucleus and electron‐dense zones. The anterior tip of the spermatozoon possesses only a few singlets. The axonemes are of a 9 + ’1’ trepaxonematan pattern and do not reach the posterior extremity of the mature spermatozoon.     

Spermiogenesis and spermatozoon of the tapeworm Ligula intestinalis (Diphyllobothriidea): phylogenetic implications

Using transmission electron microscopy, spermiogenesis and the spermatozoon ultrastructural organization are described in Ligula intestinalis (Linnaeus, 1758) (Diphyllobothriidea), a parasite of the great crested grebe Podiceps cristatus (Linnaeus, 1758). Spermiogenesis starts with the differentiation zone of 2 striated rootlets, 2 centrioles giving rise to 2 flagella, and an intercentriolar body. The latter is composed of 5 electron-dense layers separating 4 electron-lucent layers. In the early stages of spermiogenesis, an electron-dense material is present in the apical region of the differentiation zone. Later, the flagella undergo a rotation and fuse with the cytoplasmic extension in a proximo-distal process. The spermatozoon contains 2 axonemes with a 9 + "1" trepaxonematan pattern, the nucleus, the cortical microtubules, and an electron-dense zone. The spermatozoon anterior extremity in L. intestinalis is characterized by the absence of crested bodies and a ring of electron-dense cortical microtubules. Some characters of spermiogenesis and spermatozoon in L. intestinalis confirm the recent splitting of "Pseudophyllidea" into 2 new orders, i.e., Bothriocephalidea and Diphyllobothriidea. The process of spermiogenesis is similar in both orders for the "type I" of spermiogenesis and the presence of electron-dense material. However, the intercentriolar body is clearly more developed in the Diphyllobothriidea than in the Bothriocephalidea. Moreover, these 2 orders seem to differ in the presence or absence of a ring of electron-dense cortical microtubules in the anterior extremity of the spermatozoon.     

Ultrastructure of spermatids and spermatozoa in three polychaetes with modified biology of reproduction: Autolytus sp., Chitinopoma serrula,and Capitella capitata

Unlike the primitive type of spermatozoon found in most polychaetes, the spermatozoon of Autolytus has a bilateral symmetry with elongated nucleus, and the mitochondria surround the posterior part of the nucleus. A rather large disk-shaped acrosome is situated along one side of the anterior part of the nucleus. From the anterior margin of the distal centriole emerge long striated rootlets, which run along the nuclear envelope to the anterior part of the nucleus. The spermatozoon of Chitinopoma serrula has an elongated, slightly bent nucleus, a thimble-like acrosome apically on the anterior surface of the nucleus, and an elongated middle piece containing 4 rod-like mitochondria developed from spherical mitochondria surrounding the basal part of the tail flagellum. In the spermatozoon of Capitella capitata, both nucleus and middle piece are elongated compared to the primitive type. The large and conical acrosome is placed asymmetrically at the nucleus and consists of an acrosomal vesicle and subacrosomal substance. The greater part of the middle piece forms a collar around the initial part of the tail flagellum. The cytoplasm of the collar contains granular material. One or two small mitochondria lie around the 2 centrioles at the base of the nucleus.

These types of spermatozoa represent early steps in the evolution of modified spermatozoa combined with changed biology of reproduction. The modified spermatozoa are larger than the primitive ones.     

Spermiogenesis and sperm in Mesostoma viaregginum (Plathelminthes, Rhabdocoela): an ultrastructural study to infer sperm orientation

Spermiogenesis in Mesostoma viaregginum begins with the formation of a zone of differentiation containing striated rootlets, two centrioles, and an intercentriolar body in-between. These centrioles generate two parallel free-flagella with the 9+“1” pattern of the Trepaxonemata growing out in opposite directions. Spermatid differentiation is characterised by a 90° latero-ventral rotation of flagella and a subsequent disto-proximal centriolar rotation, with a distal cytoplasmic projection. The former rotation involves the compression of a row of cortical microtubules and allows recognising a flagellar side and an aflagellar side in the late spermatid and in the mature spermatozoon. At the end of the differentiation, centrioles and microtubules lie parallel to the spermatid axis. The disto-proximal centriolar rotation is proposed as a synapomorphy for the Rhabdocoela. The modifications of the intercentriolar body during spermiogenesis and the migration of the nucleus and the centrioles towards the cytoplasmic distal projection are also described. The mature spermatozoon of M. viaregginum is filiform and tapered at both ends and presents many features found in the Rhabdocoela gametes. The nucleus disappears before the flagellar insertion and a density gradient of mitochondria is observed along the sperm axis. The anterior end of the spermatozoon of M. viaregginum is characterised by a tapering capped by a membrane expansion. This study has enabled us to describe precisely the orientation of spermatozoa in the Rhabdocoela in general: the centriolar extremity is proposed as the anterior one for the Rhabdocoela.     

Ultrastructural study of the spermatozoon of Heterolebes maculosus (Digenea,Opistholebetidae), a parasite of the porcupinefish Diodon hystrix (Pisces,Teleostei)

This paper describes the ultrastructure of the mature spermatozoon of Heterolebes maculosus. It is the first study of this kind concerning the Opistholebetidae (Platyhelminthes, Digenea). The ultrastructural elements observed in the spermatozoon are: two axonemes with 9+“1” pattern of Trepaxonemata and their attachment zones, two mitochondria, a nucleus, cortical microtubules, external ornamentation of the plasma membrane and spine-like bodies. The number and the disposition of cortical microtubules, the organisation of 11 cortical microtubules disposed in semi-circle around the first mitochondrion in the external ornamentation region and the organisation of the posterior part of the spermatozoon are discussed. Three principal types of posterior part of digenean spermatozoa are proposed. The similarity between the spermatozoon of the Opistholebetidae H. maculosus and Opecoelidae enables us to confirm that these two families are closely related.     

Ultrastructure of the spermatozoon of Parachristianella trygonis Dollfus, 1946 (Trypanorhyncha: Eutetrarhynchidae)

The ultrastructural organization of the mature spermatozoon of the trypanorhynch cestode Parachristianella trygonis is described by transmission electron microscopy. The spermatozoon is a long and filiform cell, tapered at both ends, lacking both mitochondrion and crested bodies. Its cytoplasm contains 2 axonemes of the 9+'1' pattern of the Trepaxonemata longitudinally displaced, parallel cortical microtubules, the nucleus and glycogen in form of both alpha-glycogen rosettes and beta-glycogen particles. The anterior extremity of the spermatozoon is characterized by the presence of an arclike row of up to 10 parallel cortical microtubules that partially surround the first axoneme. The present study emphasizes the ultrastructural similarity between mature spermatozoa of all 4 trypanorhynchs that have been studied to date. Nevertheless, several features, i.e., the characteristics of spermatozoa extremities, the absence of crested bodies, and the possible presence of an arclike layer of cortical microtubules, need a more thorough analysis or confirmation in some of these species.     

Ultrastructural study of the male gamete of Pleurogonius truncatus Prudhoe, 1944 (Platyhelminthes, Digenea, Pronocephalidae) parasite of Eretmochelys imbricata (Linnaeus, 1766)

In Pronocephaloidea, the spermatozoa of only two species have been studied today. Because of this, we present in this work data concerning to a third specie, Pleurogonius truncatus Prudhoe, 1944. The mature spermatozoon of P. truncatus possesses two axonemes with the 9+"1" pattern typical of Trepaxonemata, mitochondrion, nucleus, parallel cortical microtubules, spinelike bodies, cytoplasmic expansion and an external ornamentation of the plasma membrane. A particularity of the spermatozoon of P. truncatus is in the ultrastructure of the anterior spermatozoon extremity with only cortical microtubules and ornamentation of the plasma membrane. This type of anterior extremity has never been described until today in Pronocephaloidea. On the other hand, the ultrastructure of the posterior extremity of the spermatozoon confirms that already described in Pronocephalidae.     

Ultrastructure of the biflagellate spermatozoon of tomopteris helgolandica greef, 1879 (annelida,polychaeta)

The spermatozoon of the polychaete Tomopteris helgolandica is of an aberrant type with two flagella, each measuring about 40μm. The nucleus is roughly conical and weakly bent. At the anterior end it is rounded and covered only by the nuclear and plasma membranes. Membraneous, electron-dense structures are applied laterally to the nucleus. These structures may have a helical arrangement. The middle piece contains about ten mitochondria, two centrioles, and two centriolar satellite complexes. The centriolar regions are connected with the posterior part of the nucleus. The axonemes of the two tail flagella lack the usual central complex with central tubules, radial spokes, or related structures. No arms seem to be present on the A tubules of the doublets. In the middle piece the tail flagella are surrounded by invaginations of the plasma membrane forming flagellar canals. The sperm has a bilateral symmetry whereas the primitive sperm has a radial symmetry. The occurrence of two tail flagella in this spermatozoon has no phylogenetical connection with biflagellate spermatozoa in other animal groups. A series of mutations has resulted in the development of two flagella emerging from the two centrioles, the lack of a central complex in the axoneme, and the lack of a typical acrosome. In the Polychaeta, sperm structure is generally more related to function that to phylogenetics. During swimming the spermatozoon of Tomopteris rotates around its longitudinal axis.