Primary projections of the trigeminal nerve in two species of sturgeon: Acipenser oxyrhynchus and Scaphirhynchus platorynchus

Horseradish peroxidase histochemical studies of afferent and efferent projections of the trigeminal nerve in two species of chondrostean fishes revealed medial, descending and ascending projections. Entering fibers of the trigeminal sensory root project medially to terminate in the medial trigeminal nucleus, located along the medial wall of the rostral medulla. Other entering sensory fibers turn caudally within the medulla, forming the trigeminal spinal tract, and terminate within the descending trigeminal nucleus. The descending trigeminal nucleus consists of dorsal (DTNd) and ventral (DTNv) components. Fibers of the trigeminal spinal tract descend through the lateral alar medulla and into the dorsolateral cervical spinal cord. Fibers exit the spinal tract throughout its length, projecting to the ventral descending trigeminal nucleus (DTNv) in the medulla and to the funicular nucleus at the obex. Retrograde transport of HRP through sensory root fibers also revealed an ascending bundle of fibers that constitutes the neurites of the mesencephalic trigeminal nucleus, cell bodies of which are located in the rostral optic tectum. Retrograde transport of HRP through motor root fibers labeled ipsilateral cells of the trigeminal motor nucleus, located in the rostral branchiomeric motor column.     

Developmental study of the shortened spinal cord in the adult tiger puffer fish, Takifugu rubripes (Teleostei)

ABSTRACT The spinal cords of vertebrates are generally divided into the cord proper and the minute filum terminale. While the spinal cord extends the entire length of the vertebral canal in the adult tiger puffer, Takifugu rubripes, the cord proper is greatly reduced in length and almost all of the canal is occupied by the filum terminale, which is tape-like rather than thread-like. The dorsal and ventral roots of the spinal nerves extend, respectively, above and below the filum terminale; as a whole, these form a massive cauda equina. Supramedullary cells are found in the rostral half of the medulla oblongata caudal to the cerebellum. In 4-mm long tiger puffers, the spinal cord is cylindrical and supramedullary cells are found in the rostral half of the cord. In 7-mm puffers, the longitudinally arranged ventral roots appear ventrally in the middle portion of the spinal cord. In 15-mm puffers, the dorsal and ventral roots run longitudinally along the spinal cord and have noticeably increased in number. Supramedullary cells are located in the rostral 15% of the cord. In 21-mm puffers, the spinal cord in large part becomes dorsoventrally flattened. In 30-mm puffers, the spinal cord becomes much flatter, and supramedullary cells now are located mainly in the medulla oblongata. These observations indicate that formation of the shortened spinal cord proper is due to at least two developmental processes. First, the elongation of the spinal cord proper is remarkably less than that of the vertebral canal. Second, the bulk of the spinal cord proper is translocated to the cranial cavity, where it is transformed into part of the medulla oblongata.     

Anatomy,neurophysiology and functional aspects of the nucleus isthmi in salamanders of the family Plethodontidae

Summary Tongue-projecting plethodontid salamanders have massive direct ipsilateral retinal afferents to the tectum opticum as well as a large and well developed nucleus isthmi. Retrograde staining revealed two subnuclei: A ventral one projecting to the contralateral tectal hemisphere and a dorsal one projecting back to the ipsilateral side. The isthmic nuclei show a retinotopic organization, which is in register with that of the tectum. Electrophysiological recordings from nucleus-isthmi neurons revealed response properties that are very similar to those found in tectal neurons. Thus, there is no substantial processing of tectal neural activity in the nucleus isthmi. Measurements of peak latencies after electrical and light stimulation suggest the continuous coexistence of 4 representations of the visual field in the tectum mediated by (1) the contralateral and (2) the ipsilateral direct retinal afferents, (3) the uncrossed and (4) the crossed isthmo-tectal projection. (1) and (2) originate at the same moment in the retina and arrive simultaneously in the tectum. It is assumed that in plethodontid salamanders with massive ipsilateral retino-tectal projections depth perception based on disparity cues is achieved by comparison of these images.Representations mediated by (3) and (4) arriving in the tectum at the same time as (1) and (2) originate 10–30 ms earlier in the retina. It is hypothesized that these time differences between (1)/(2) and (3)/(4) are used to calculate three-dimensional trajectories of fast-moving prey objects.Abbreviations EL edge length - FDA fluoresceine dextranamine - RDA tetramethylrhodamine dextranamine - RF receptive field     

Distribution of choline acetyltransferase and acetylcholinesterase in the nervous system of the dog

Choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity were determined in 23 selected parts of the dog CNS and 4 parts of the peripheral nervous system. Maximum ChAT activity was found in the caudate nucleus and the ventral roots of the spinal cord. High activity was also present in the thalamus, the pons, the cerebral cortex, the medulla oblongata, the ventral spinal horns and the sciatic nerve. The lowest activity was measured in the cerebellum, the dorsal cord roots and the spinal ganglia. Maximum AChE activity was found in the caudate nucleus and the cerebellum. Relatively high activity was also present in the thalamus, the pons, the medulla oblongata, the grey matter of the spinal cord and the spinal ganglia. The lowest AChE activity was measured in the ventral and dorsal spinal roots.     

Afferent and efferent components of the facial nerve in a frog,Rana pipiens

Summary The seventh cranial nerve in Rana pipiens is a slender nerve with limited peripheral distribution. We investigated the afferent and efferent components of this nerve by labeling its major branch, the hyomandibular, with horseradish peroxidase. The efferent portion of the seventh nerve originates from a small cell group in the upper medulla which contains two subdivisions. Afferent fibers carried in nerve VII travel in the solitary tract and the dorsolateral funiculus. The solitary component consists of a small number of ascending fibers that reach the level of the trigeminal nucleus and a large descending component that terminates slightly caudal to the obex in the commissural nuclei of the solitary complex. Afferent fibers also descend in the dorsolateral funiculus; many of these fibers cross dorsal to the central canal in the lower medulla. Most of the fibers in the dorsolateral funiculus terminate in the ipsilateral and contralateral dorsal horns and in nuclei of the dorsal column. A few ipsilateral fibers reach lower thoracic levels of the spinal cord.     

Projections of the optic tectum and the mesencephalic nucleus of the trigeminal nerve in the tegu lizard (Tupinambis nigropunctatus)

Summary Fibers undergoing Wallerian degeneration following tectal lesions were demonstrated with the Nauta and Fink-Heimer methods and traced to their termination. Four of the five distinct fiber paths originating in the optic tectum appear related to vision, while one is related to the mesencephalic nucleus of the trigeminus. The latter component of the tectal efferents distributes fibers to 1) the main sensory nucleus of the trigeminus, 2) the motor nucleus of the trigeminus, 3) the nucleus of tractus solitarius, and 4) the intermediate gray of the cervical spinal cord.The principal ascending bundle projects to the nucleus rotundus, three components of the ventral geniculate nucleus and the nucleus ventromedialis anterior ipsilaterally, before it crosses in the supraoptic commissure and terminates in the contralateral nucleus rotundus, ventral geniculate nucleus and a hitherto unnamed region dorsal to the nucleus of the posterior accessory optic tract.Fibers leaving the tectum dorso-medially terminate in the posterodorsal nucleus ipsilaterally and the stratum griseum periventriculare of the contralateral tectum. The descending fiber paths terminate in medial reticular cell groups and the rostral spinal cord contralaterally and in the torus and the lateral reticular regions ipsilaterally. The ipsilateral fascicle also issues fibers to the magnocellular nucleus isthmi.     

Retinofugal projections in salamanders of the family Plethodontidae

Summary A comparison of the retinofugal projections in 14 species of plethodontid salamanders by means of the horseradish peroxidase (HRP) technique revealed almost identical contralateral projections. In all species studied three optic tracts were found. Behind the chiasma opticum the basal optic tract runs to the peduncle region, there forming the basal optic neuropil. The marginal optic tract courses from the chiasma over the thalamus to the tectum opticum where it covers the entire surface. In the anterior thalamus the marginal optic tract innervates the neuropil Bellonci-pars lateralis and the corpus geniculatum thalamicum, and more caudally the neuropil posterior thalami. The medial optic tract supplies the neuropil Bellonci-pars lateralis and pars medialis in the anterior thalamus from where it runs medial to the marginal optic tract as a separate tract to the uncinate field in the posterior thalamus.The ipsilateral projections show differences among the species studied, although the global organization remains constant. The differences mainly concern the marginal optic tract which varies from being weakly labeled and restricted to the rostral part of the tectum opticum, to being heavily labeled and innervating the entire tectum to its caudal edge. Species with the heaviest ipsilateral projections all belong to the plethodontid tribe Bolitoglossini, all of which show direct development, a highly projectile tongue, rather frontally oriented eyes and excellent depth perception. In these species the thalamic ipsilateral projection areas are equal in size and shape to the contralateral one. The ipsilateral projections to the tectum show two distinct layers, a superficial and a deep one, which intermingle with the contralateral projections. The two other ipsilateral tracts do not differ significantly among the plethodontid species: the medial optic tract is always heavily and the basal optic tract always weakly labeled.     

PKCγ免疫反应定位小鼠皮质脊髓束的实验研究

目的探讨显示皮质脊髓束在成年小鼠脑和脊髓中定位分布的简便有效方法。方法运用蛋白激酶Cγ（PKCγ）免疫组织化学染色法,观察成年ICR小鼠脑和脊髓中皮质脊髓束的定位和分布情况。结果PKCγ免疫阳性产物分布于大脑运动皮层第V层锥体细胞胞体和轴突中,锥体细胞的阳性纤维经内囊、中脑大脑脚底、脑桥基底部、下行至延髓锥体中。在延髓下段,PKCγ阳性纤维经锥体交叉后进入对侧脊髓灰质后联合背侧,形成背侧皮质脊髓束,在脊髓白质的后索腹侧深层下行,至骶髓3-4节段以下逐渐消失。在整个脊髓前索和外侧索中未见有PKCγ阳性纤维。结论PKCγ特异地表达于脊髓后索皮质脊髓束中,提示PKCγ免疫组织化学法是一种显示和观察皮质脊髓束精确定位的有效方法。     

Connectivity of the salamander pretectum: an in-vitro (whole-brain) intracellular tracing study

The amphibian optic tectum and pretectum have been analyzed in detail anatomically and physiologically, and a specific model for tecto-pretectal interaction in the context of the visual guidance of behavior has been proposed. However, anatomical evidence for this model, particularly the precise pattern of pretecto-tectal connectivity, is lacking. Therefore, we stained pretectal neurons intracellularly in an in-vitro preparation of the salamanders Plethodon jordani and Hydromantes genei. Our results demonstrate that the projections of neurons of the nucleus praetectalis profundus are divergent and widespread. Individual neurons may project divergently to telencephalic (ipsilateral amygdala and striatum), diencephalic (ipsi-and contralateral thalamus, contralateral pretectum), and mesencephalic (ipsi- and contralateral tectum and tegmentum) centers, and to the ipsi- and contralateral medulla oblongata and rostral spinal cord. The projection of pretectal cells to the optic tectum is bilateral; axonal structures do not show discernible patterns and are present in all layers of the superficial white matter. A classification of pretectal neurons on the basis of axonal termination pattern or dendritic arborization has not been possible. Our results do not support the hypothesis that a distinct class of pretectal neurons projects to a particular subset of tectal cells. Rather, the pretectum appears to influence the tectum indirectly, acting either on retinal afferents or modulating inhibitory interneurons.     

Neurons of the medulla oblongata and hypothalamus projecting on the sympathetic neurons of the ventral horn of the spinal cord]

Connections of the neurons of the spinal cord ventral horn with the structures, situating above have been investigated. After injection of uranyl acetate into the TIII segment of the spinal cord, labelled neurons are found in various reticular nuclei of the medulla oblongata. At the level of the roots of the XII pair of the cranial nerves they are revealed in the reticular paramedian, ventral, parvocellular and lateral nuclei. The formations mentioned participate in regulation of the cardio-vascular system. More rostral (2 and 4 mm relatively to the roots of the XII pair of the cranial nerves) the neurons are observed in the reticular giant cellular nucleus, in nuclei of the raphe and in the group of the P-substance reactive neurons. Besides, labelled neurons are revealed in the posterior, lateral fields and in the dorso- and ventromedial nuclei of the hypothalamus.     

Histoenzmological mapping of ATPase and 5-nucleotidase in the spinal cord and medulla oblongata of hedgehog (Paraechinus micropus)

The contribution presented deals with the distribution of adenosine triphosphatase (ATP-A) and 5-nucleotidase (AMP-A) in the spinal cord and medulla oblongata of hedgehog. The highlights of this study are: (1) AMP-A activity is stronger in neuropil than in neurons, in all the areas of spinal cord and medulla oblongata. In the nerve cells the enzyme is localized at the peripheries of the neurons, whereas the cytoplasm and nuclei are completely free from enzymatic activity. Reaction in blood vessels is quite high both in gray and white matter. (2) ATP-A activity is seen mainly at the peripheries of the neurons. The neuropil activity varies from mild to intense. Reaction in blood vessels is quite strong in all the areas. (3) Fibrous bundles and tracts are negative for both the enzymes. (4) In general, the activity of ATP-A and AMP-A is strongest in cranial nerve nuclei, irrespective of their sensory or motor nature. The distribution of these enzymes has been correlated with the functions of various nuclei of spinal cord and medulla oblongata in hedgehog, and compared with other mammals.     

The terminal abdominal ganglion of the wood cricket Nemobius sylvestris

The abdominal cerci of the wood cricket, Nemobius sylvestris, are covered by a variety of hair‐like sensilla that differ in length, thickness, and articulation. Fillings from the cercal nerves with cobalt chloride and fluorescent dyes revealed the projection of sensory axons into the terminal abdominal ganglion of the ventral nerve chain. Two projection areas on each side of the terminal abdominal ganglion midline could be identified: a posterior cercal glomerulus and an anterior bristle neuropil. Axons from some cercal sensilla ascend through the connectives to reach the metathoracic ganglionic mass. As their axons pass through each segmental abdominal ganglion, they project medial arborization. Cross‐sections of the terminal abdominal ganglion and retrograde fills with cobalt chloride and fluorescent dyes from connectives revealed several small cells and seven pairs of giant ascending interneurons organized symmetrically. Giant somata are located contralateral to their axons (diameters between 20 and 45 μm). The cercal projections overlap extensively with the dendritic fields of the giant interneurons. In the terminal abdominal ganglion, we identified nine longitudinal tracts, two major tracts, and seven smaller ones. The functional implications of the neuranatomical organization of the system are discussed on a comparative basis. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Dil对鱼类视觉传导通路的形态学研究

目的：应用Dil染色晶体研究罗非鱼脑、脑神经及其视觉传导路的形态分布。方法：罗非鱼6只,体长12．16ctn,进行灌注固定后,在外科显微镜下开颅并确认脑和脑神经根,分别于双侧视神经植入Dil染色晶体。37℃恒温箱放置3个月,待Dil染色晶体扩散后,取出植入Dil染色晶体的视神经和脑,再根据神经走向切片,通过荧光显微镜观察Dil染色晶体在视觉传导路的形态分布。结果：①外科显微镜下可以观察到罗非鱼的脑分为端脑、中脑、间脑、小脑和延脑5部分,并同时观察到10对脑神经。②右侧视神经植入Dil染色晶体后,均可见到标记的右侧视神经纤维,行向后内,穿经视神经管入颅,逐步靠近左侧视神经,进一步行向后内,经过左侧视神经上方,进行完全交叉,形成视交叉,再经左侧视柬连于左侧间脑视盖。结论：罗非鱼的脑分为5部分,脑神经只有10对。罗非鱼视交叉属于完全性交叉,视觉中枢可能位于间脑视盖内。     

The area octavo-lateralis in Xenopus laevis

Summary Central projections of afferents from the lateral line nerves and from the individual branches of the VIIIth cranial nerve in Xenopus laevis and Xenopus mülleri were studied by the application of HRP to the cut end of the nerves.Upon entering the rhombencephalon, the lateral line afferents form a longitudinal fascicle of ascending and descending branches in the ventro-lateral part of the lateral line neuropile. The fascicle exhibits a topographic organization, that is not reflected in the terminal field of the side branches. The terminal field can be subdivided into a rostral, a medial and a caudal part, each of which shows specific branching and terminal pattern of the lateral line afferents. These different patterns within the terminal field are interpreted as the reflection of functional subdivisions of the lateral line area. The study did not reveal a simple topographic relationship between peripheral neuromasts and their central projections.Two nuclei of the alar plate with significant lateral line input were delineated: the lateral line nucleus (LLN) and the medial part of the anterior nucleus (AN). An additional cell group, the intermediate nucleus (IN), is a zone of lateral line and eighth nerve overlap, although such zones also exist within the ventral part of the LLN and the dorsal part of the caudal nucleus (CN). Six nuclei which receive significant VIIIth nerve input are recognized: the cerebellar nucleus (CbN), the lateral part of the anterior nucleus, the dorsal medullary nucleus (DMN), the lateral octavus nucleus (LON), the medial vestibular nucleus (MVN) and the caudal nucleus (CN).All inner ear organs have more than one projection field. All organs project to the dorsal part of the LON and the lateral part of the AN. Lagena, amphibian papilla and basilar papilla project to separate regions of the dorsal medullary nucleus (DMN). There is evidence for a topographic relation between the hair cells of the amphibian papilla (AP) and the central projections of AP fibers. The sacculus projects extensively to a region between the DMN and the LON. Fibers from the sacculus and the lagena project directly to the superior olive. Fibers from the utriculus and the three crista organs terminate predominantly in the medial vestibular nucleus (MVN) and in the adjacent parts of the reticular formation, and their terminal structures appear to be organotopically organised. Octavus fiber projections to the cerebellum and to the spinal cord are also described.     

Ascending spinal systems in the nurse shark,Ginglymostoma cirratum

Summary The ascending spinal systems in the nurse shark were studied after spinal hemisections by use of the Nauta and Fink-Heimer techniques. The dorsal funicular fibers form a single bundle issuing fibers to the gray substance of the spinal cord, the dorsal funicular nucleus, and the vestibular complex. Some dorsal funicular fibers also appear to contribute to the spinocerebellar tract.The degenerated lateral funicular fibers are segregated into three fasciculi issuing fibers medially as they ascend through the brainstem. The largest target of these fibers is the reticular formation, but diffusely organized axons also reach 1) the gray matter of the spinal cord, 2) the dorsal motor nucleus of the vagus, 3) the nucleus A of the medulla oblongata, 4) the central gray substance of the brainstem, 5) the cerebellar cortex, 6) the cerebellar nucleus, 7) the nucleus intercollicularis, 8) the mesencephalic tectum, and 9) the dorsal thalamus. At the latter site the spinal input appears to partly overlap with the visual input.The results, compared with the strikingly similar findings in other classes of vertebrates, indicate that all vertebrate groups apparently have the same basic components of ascending spinal projections.     

Central afferent pathways of long hair sensilla in the ventral nerve cord of the Indian black scorpion,Heterometrus fulvipes Koch

Histological and electrophysiological studies of identified long hair sensilla (LHS) have provided information on primary afferent fibre pathways in the ventral nerve cord of the Indian black scorpion, Heterometrus fulvipes.Cobalt-filling of single LHS on the metasoma showed that sensory axons enter the respective segmental ganglion, ascend ipsilaterally through the next anterior ganglia and terminate in a 4th ganglion. In each ganglion, these plurisegmental fibres give off collateral branches that terminate in the ganglionic neuropil. Fibres entering heterolateral connectives were not found.Recordings from peripheral nerves after deflections of a hair showed single or multiple spike discharges. A single spike could be recorded from ipsilateral anterior connectives of the ventral nerve cord, indicating a through-conductance of the sensory pathways. Strong deflections of a single hair activated several ipsilateral and fewer contralateral ascending interneurons and some segmentai motor neurons. Behavioral studies demonstrate the mechanoreceptive function of the LHS.The present study provides evidence in support of the notion that sensory afferents of the postabdomen in the scorpion bring about rapid, co-ordinated intersegmental movements of the multisegmented tail of the scorpion.Abbreviations CNS central nervous system - LHS long hair sensillum - TR trichobothria     

Olfactory and vomeronasal projections and the pathway of the nervus terminalis in ten species of salamanders

Summary Primary olfactory and vomeronasal projections as well as the pathway of the nervus terminalis were studied in 10 representative species of salamandrid and plethodontid salamanders by means of injections of horseradish peroxidase and examination of whole-mount preparations. Olfactory projections are very similar in the different urodeles, but vomeronasal projections differ in shape and number of termination fields. Whereas the direct-developing Plethodontini and Bolitoglossini reveal only one or two fields, the salamandrid species and the members of the plethodontid tribes Desmognathinae and Hemidactyliini, all possessing an aquatic larval stage, exhibit several vomeronasal projection fields. In all species examined centrifugal axons of the nervus terminalis leave the olfactory projection area ventrocaudally and terminate in the preoptic region and the hypothalamus.Abbreviations COM. ANT commissura anterior - DGL displaced glomeruli - HY hypophysis - HYTH hypothalamus - LF lateral fibers of the nervus terminalis - ME medulla oblongata - MF medial fibers of the nervus terminalis - Nt nervus terminalis - Npo nucleus praeopticus     

Ascending and descending efferent pathways of the midbrain reticular formation of the cat (radioautographic study)

By means of the anterograde axoplasmic transport technique for a mixture of labelled aminoacids (3H-leucine and 3H-proline), ascending and descending systems of the reticular formation fibers in the cat mesencephalon have been studied. Projections from the mesencephalon reticular formation (MRF) ascend to the subthalamus, lateral, dorsal and periventricular hypothalamus, to the periventricular nuclei of the midline and to the intralaminar nuclei of the thalamus. The descending pathways project to the grey substance surrounding the aqueduct of cerebrum, locus coeruleus, parabrachial region and reticular formation of the pons and medulla oblongata. The projections to the reticular nucleus of the thalamus, ventral nucleus of the external geniculate body and superior colliculi arise from the dorsal half of the MRF, and projections to the striatum, lateral reticular nucleus of the medulla oblongata--from its ventral half. Most of the structures are reciprocally connected with the MRF.     

Neonatal 6-hydroxydopamine treatment: noradrenaline levels and in vitro 3Hcatecholamine synthesis in discrete brain regions of adult rats

Endogenous noradrenaline levels are elevated in medulla oblongata, mesencephalon, pons and thalamus of adult rats which had been treated with 6-hydroxydopamine on days 1, 2, 8 and 15 after birth. Levels in spinal cord, cerebellum, hippocampus/amygdala and cortex are depressed, whereas no significant changes are observed in striatum, hypothalamus and medulla spinalis. The rate at which medulla oblongata synthesizes tritiated noradrenaline and dopamine from tritiated tyrosine $\text{in}$$\text{vitro}$ is markedly enhanced. No effect was apparent on catecholamine synthesis in hypothalamus. Tritiated noradrenaline synthesis, but not tritiated dopamine synthesis, in the cortex is depressed. These results support the view that neonatal 6-hydroxydopamine treatment causes a degeneration of noradrenaline nerve terminals in the cortex and induces an increase in noradrenaline terminals in the medulla oblongata.     

Morphology of the brain and sense organs in the snailfish Paraliparis devriesi: Neural convergence and sensory compensation on the Antarctic shelf

The Antarctic snailfish Paraliparis devriesi (Liparidae) is an epibenthic species, inhabiting depths of 500–650 m in McMurdo Sound. Liparids are the most speciose fish family in the Antarctic Region. We examine the gross morphology and histology of the sense organs and brain of P. devriesi and provide a phyletic perspective by comparing this morphology to that of four scorpaeniforms and of sympatric perciform notothenioids. The brain has numerous derived features, including well-developed olfactory lamellae with thick epithelia, large olfactory nerves and bulbs, and large telencephalic lobes. The retina contains only rods and exhibits a high convergence ratio (82:1). Optic nerves are small and nonpleated. The tectum is small. The corpus of the cerebellum is large, whereas the valvula is vestigial. The rhombencephalon and bulbospinal junction are extended and feature expanded vagal and spinal sensory lobes as well as hypertrophied dorsal horns and funiculi in the rostral spinal cord. The lower lobes of the pectoral fins have taste buds and expanded somatosensory innervation. Although the cephalic lateral line and anterior lateral line nerve are well developed, the trunk lateral line and posterior lateral line nerve are reduced. Near-field mechanoreception by trunk neuromasts may have been compromised by the watery, gelatinous subdermal extracellular matrix employed as a buoyancy mechanism. The expanded somatosensory input to the pectoral fin may compensate for the reduction in the trunk lateral line. The brains of P. devriesi and sympatric notothenioids share well-developed olfactory systems, an enlarged preoptic-hypophyseal axis, and subependymal expansions. Although the functional significance is unknown, the latter two features are correlated with habitation of the deep subzero waters of the Antarctic shelf. J. Morphol. 237:213–236, 1998. © 1998 Wiley-Liss, Inc.