PYROSOMA FURTHER DEVELOPMENT OF THE CYATHOZOOID. 403 



degree of development, the Cyathozooid begins gradually to degenerate. 

 It can then be seen as an oval body (Fig. 196, c) gradually decreasing 

 in size, at the centre of the young tetrazooid colony. Its atrial 

 aperture (cl) closes, and it is gradually 

 absorbed till not a trace of it is left. 

 KOWALEVSKY thought that the atrial cavity 

 of the Cyathozooid persisted as the common 

 cloacal cavity of the whole colony ; this, 

 however, is in opposition to the above 

 account by SALENSKY. In accepting the 

 latter account we must assume that the 

 common cloacal cavity of the colony repre- 

 sents a depression of the surface of the 

 cellulose mantle which appears later. 



We have still briefly to describe the way 

 in which the structure of the Cyathozooid 

 is to be interpreted and to compare it with 

 that of a typical Tunicate (Fig. 199), and 

 in connection with this we must explain 

 why, together with KOWALEVSKY and 

 SALENSKY, we regard the ectodermal depres- 

 sion in the Cyathozooid just mentioned as 

 the atrial cavity (c). The fact that the 

 peribranchial tubes open into this cavity is 

 in favour of interpreting it in this way, and 

 this view is still further confirmed by the position of the nervous 

 system with relation to this invagination. If it is to be regarded as 

 the inhalent aperture, the ciliated pit (fl) of the central nervous 

 system would have to 

 be directed towards it, 

 but this is not the 

 case. The ciliated pit 

 forms the part of the 

 central nervous system 

 (g) which is turned 

 away from the invagi- 

 nation. A comparison 

 of a diagram illustrat- 

 ing the relative positions of the most important organs in the body 

 of a solitary Salp (Fig. 199 B) with a diagram of a Cyathozooid 



FIG. 197. Later stage iii 

 the development of the 

 central nervous system of 

 the Cyathozooid (after 

 SALENSKY ; cf. Fig. 194 

 B, g). d, wall of pharynx ; 

 dh, enteric cavity ; fl, cili- 

 ated pit ; y, ganglion ; sn , 

 lateral nerves. 



FIG. 198. Transverse section through the developing 

 test of the Cyathozooid in the stage depicted in Fig. 

 195 (after SALENSKY). c, cellulose substance ; ec, ecto- 

 derm ; ms, meseuchyme-cells ; ms', mesenchyme-cells 

 forming hexagonal pattern in test (cf. Fig. 196). 



