450 



TUNICATA. 



In the Mediterranean the larvae of Amaroucium hatch out in 

 winter (MAURICE and SCHULGIN, No. 39). After becoming attached, 

 they multiply by transverse fission, and throughout the whole summer 

 produce a succession of generations exclusively asexually, and in this 

 way the colony grows. Towards the beginning of winter the youngest 

 zooids cease to reproduce in this manner and develop sexual organs. 

 The older asexual individuals of the colony die off, and their dis- 

 integrated remains are, according to MAURICE'S observations on 

 Fragaroides (No. 40), taken in and digested by the mantle-cells 

 which function as phagocytes (p. 356). 



KOWALEVSKY investigated the asexual reproduction that takes place 



in the primary individuals of the 

 colony which result from the larva. 

 He found in them (Fig. 227 A) three 

 body-regions; thorax (a), abdomen 

 (b) and post-abdomen (c). The post- 

 abdomen is the elongated posterior 

 region of the body in which the 

 genital organs develop in the herma- 

 phrodite individual (hence the term 

 " bourgeonnement ovarien"). In a 

 cross-section (Fig. :?25) through the 

 post-abdomen of an individual about 

 to commence multiplying asexually, 

 beneath the ectoderm, longitudinal 

 muscle-bundles (m) are seen em- 

 bedded in an extensive layer of 

 mesoderm-cells (ms), these latter 

 seeming to be filled with reserve 

 nutritive material. The primary body-cavity (b, b') which is con- 

 tinued upward into the thorax and the abdomen, appears divided by 

 a transverse partition- wall (s) into a dorsal half (b) and a ventral half 

 (>'). The partition-wall itself is hollow, and is nothing more than a 

 flat diverticulum of the branchial sac arising from the latter immedi- 

 ately behind the posterior end of the endostyle, between it and the 

 entrance of the oesophagus, which runs back through the whole of the 

 post-abdomen and ends blindly near its posterior end. Here lies the 

 heart (Fig. 227 A, h) curved into a crescent round the posterior end of 

 the entodermal process just mentioned. This entodermal diverticulum 

 is identical with the tube in Clavelina called by VAN BENEDEN and 

 JULIN (No. 10) the epicardial tube (see Fig. 173 C, ey, p. 375). It 



FIG. 225. Transverse section 

 through the post-abdomen of 

 Amtttrmtcium (after KOWALEVSKY). 

 b, upper, b', lower blood-sinus ; 

 ec, ectoderm ; m, muscle-fibres in 

 transverse section ; ms, mesoderm- 

 cells ; s, partition-wall (epicardial 

 sac). 



