PYROSOMA — -FURTHER DEVELOPMENT OK THE CYATHOZOOID. 



401 



ment of the ganglion (</). This part gives off two lateral processes, 



the rudiments of the lateral nerves (sn) which clasp the enteric canal 

 (-/) laterally and end in the lower wall of the atrial invagination. 

 Within the ganglionic rudiment of the Cyathozooid there is never any 

 development of punctated nervous tissue (Punktsubstanz). 



The development of the 

 hea/rt has already been de- cl 



scribed (Fig. 193, pc). ^ J-JL^ 



The ectoderm of the 

 Cyathozooid yields the cel- 

 lulose test of the young 

 colony. The secretion of 

 this layer begins even before 

 the circumcresence of the 

 yolk-sphere by the Cyatho- 

 zooid is fully completed 

 (Fig. 194 A, m). The area 

 occupied by the Cyathozooid 

 is then indicated by the 

 extension of the cellulose 

 mantle which, at a later 

 period, encloses the four 

 primary Ascidiozooids (Figs. 

 194, 195). The ectoderm 

 of the Ascidiozooids does 

 not, according to Salensky, 

 take part in the formation 

 of the cellulose investment 

 of the young colony, which 

 is yielded exclusively by the 

 • \Mthozooid. The process 

 by which this mantle is 

 secreted agrees pretty 

 closely with that described by Kowalevsky for the Ascidiacea (p. 355) 

 According to Salensky, single mesoderm-cells (mesenchyme-cells), 

 wandering through the ectoderm, come to lie on its external surface 

 (Fig. 198, ms) and are the cells found later in the test. The secretion 

 of the latter which now takes place proceeds from the ectoderm-cells 

 (ec), each cell giving off externally a perpendicular plate-like process, so 

 that the cellulose layer which is secreted seems broken up by these 

 processes into separate prisms. At the same time, the mantle-cells 



DD 



PlG. 195. — 'Later ontogenetic stage of a tetrazooid 

 colony of Pyrosoma (after Salensky). The 

 greater part of the food-yolk has been absorbed. 

 The cellulose mantle of the Cyathozooid has 

 grown round the Ascidiozooids. cl, atrial 

 aperture of the Cyathozooid ; <l, alimentary 

 canal ; en, endostyle ; i, inhalent orifice ; ks, 

 gill-clefts ; m, mantle ; x, cell-lamellae of the 

 cellulose mantle ; z, remains of the cell-zone. 



