414 TUNICATA. 



The body - musculature, which is only slightly developed in 

 Pyrosoma, arises from mesenchy me -cells arranged in bands. The 

 fibrils of contractile substance are arranged round the periphery of 

 these cell-bands which, in cross-section, appear triangular and extend 

 in a radial manner towards the interior of the muscle -bundle. 



As may be seen from Fig. 196, p. 402, the four primary Ascidio- 

 zooids of the young colony are placed radially to the principal axis 

 in such a way that their branchial apertures appear to lie equatorially 

 on the outer surface. The centre of the colony is at first occupied 

 by a Cyathozooid but later by the common cloacal cavity which opens 

 externally at the pole marked cl in Fig. 196. The dorsal surface of 

 each Ascidiozooid is turned towards the cloacal cavity ; its ventral 

 surface, on the contrary, seems turned towards the opposite pole. 

 Since it is here (at the end of the endostyle) that the first buds 

 become abstricted, it might be expected that the oldest Ascidiozooid 

 of the colony would lie nearest to the common cloacal aperture, while 

 the younger Ascidiozooids would occupy the posterior part of the 

 colony, that turned away from the cloacal aperture. According to 

 SEELIGER, however, this is not the case ; but the young buds, after 

 becoming separated from the parent individual, wander to its dorsal 

 side so that they become intercalated between it and the common 

 cloacal aperture. Each Ascidiozooid gives off towards the cloacal 

 aperture two mantle-vessels which, in Fig. 196 B, can already be 

 seen as dorsal processes in the neighbourhood of cl. In large colonies, 

 the four primary zooids surround the posterior pole of the colony, 

 that turned away from the cloacal aperture and their man tie -vessels 

 must consequently have the longest course. 



5. The Hemimyaria (Salpidae). 



The embryonic development of the Salpidae stands in somewhat 

 sharp contrast to that of other Tunicates. The fact that the 

 developing embryo fuses with the wall of the atrial cavity of the 

 mother and that, at the point of fusion, there develops a nutritive 

 organ known as a placenta derived in part from the remains of the 

 egg-follicle, has brought about divergent ontogenetic conditions. 

 The development is abbreviated, as it usually is where it takes place 

 within the body of the mother. Neither the larval tail nor the 

 chorda develops. It must be at once admitted that our knowledge 

 of the embryonic development of the Salpidae must not be regarded 

 as in any way complete. Even with regard to the most important 



