Acoel development supports a simple planula-like urbilaterian

Molecular approaches to the study of development and evolution have had profound effects on our understanding of the nature of the evolutionary process. Developmental biologists became intoxicated with fanciful notions of reconstructing genetic pathways of morphogenesis while evolutionary biologists were sobered by the fallacy of reconstructing organismal relationships along increasing grades of morphological complexity. Increased taxon sampling and improvements in analytical techniques are providing a new approach and are forcing biologists to move past historical biases to allow more accurate mapping of morphological and developmental characters through evolutionary time. Here, we discuss the possible developmental and morphological features of the 'urbilaterian', the triploblastic animal with anterior-posterior and dorsoventral axes and predecessor of the protostome-deuterostome ancestor. We argue that this animal, with features resembling acoelomorph flatworms, was far simpler morphologically than the protostome-deuterostome ancestor despite possessing a nearly complete eubilaterian genome. We show that the deployment of some genes expected to pattern the protostome-deuterostome ancestor is not deployed in acoels in the predicted manner and thus might have been co-opted after the evolution of the urbilaterian. We also identify the developmental changes related to gastrulation that gave rise to the urbilaterian from a simpler cnidarian-like ancestor.     

澳洲异尾涡虫（无肠目，盘旋科）新纪录及描述

本文对分布于广东省深圳市深圳湾体育中心海边(22°30'58″N,113°56'1″E)的一种褐绿色涡虫的形态、发育、行为等做了详细观测,并对其进行了分类学与动物地理方面的讨论。结果表明,此涡虫表皮内具共生的虫黄藻(Zooxanthella microadriatica),另具菌丝状分布的棕色颗粒。头部钝圆,具1对米粒状棕色眼点和1个平衡囊。尾后两侧具2条并列的尾垂,其形状和大小与涡虫尾部的舒展状态相关。雌雄同体,生殖孔2个,位于口后,雌孔在前,雄孔在后。卵巢、精巢成双,分别位于腹侧与背侧。受精囊呈哑铃型,囊内具2~12根囊管。阴道呈倒"T"形,其表皮由柱状细胞构成。精巢内精子集聚成束,向体后两侧延伸成假储精囊,汇入储精囊。储精囊环抱阴茎球。涡虫利用纤毛的摆动在水底滑动,或利用两侧身体的摆动呈蝶状游泳。喜捕食微型涡虫与轮虫,人工饲养可摄食水蚤或水溞。经比较,鉴定为无肠目盘旋科(Convolutidae)异尾属(Heterochaerus)澳洲异尾涡虫(H.australis Haswell,1905),属中国涡虫1新纪录目1新纪录种。     

An ultrastructural and cytochemical study of the digestive system in Oxyposthia praedator (Turbellaria,Acoela)

The central parenchyma in Oxyposthia praedator consists of multifunctional cells. These cells digest food material intracellularly, can effect extracellular digestion through release of digestive enzymes by cell lysis and clasmatosis, and synthesize reserve nutritional substances.     

Free-living flatworms under the knife: past and present

Traditionally, regeneration research has been closely tied to flatworm research, as flatworms (Plathelminthes) were among the first animals where the phenomenon of regeneration was discovered. Since then, the main focus of flatworm regeneration research was on triclads, for which various phenomena were observed and a number of theories developed. However, free-living flatworms encompass a number of other taxa where regeneration was found to be possible. This review aims to display and to compare regeneration in all major free-living flatworm taxa, with special focus on a new player in the field of regeneration, Macrostomum lignano (Macrostomorpha). Findings on the regeneration capacity of this organism provide clues for links between regeneration and (post-)embryonic development, starvation, and asexual reproduction. The role of the nervous system and especially the brain for regeneration is discussed, and similarities as well as particularities in regeneration among free-living flatworms are pointed out.     

Architectonics of the central nervous system of Acoela,Platyhelminthes, and Rotifera

Based on the literature and own data, consecutive stages of development of the central nervous system (CNS) in the lower Bilateria are considered-separation of brain from parenchyma, formation of its own envelopes, and development of the trunk and orthogonal nervous system. Results of histochemical (cholinergic and catecholaminergic) and immunocytochemical (5-HT-and FMRF-amid immunoreactive) studies of the CNS in representatives of Acoela, free living and parasitizing Platyhelminthes and Rotifera are considered. The comparative analysis makes it possible to describe development and complication of the initially primitive Bilateria pleux nervous system. A special attention will be paid to the Acoela phylogenesis, based on molecular-biology data and results of study of their nervous system.