PYROSOMA — THE PRIMARY TETRAZOOID COLONY. 



399 



While the four Ascidiozooids continue to increase in size and develop 

 the structure of the adult individual (Figs. 194-196), the Cyathozooid 



which lies in the midst of them gradually atrophies (Fig. 196, c). 

 Only m»w (Fig. L96 B) does the colony, which is enveloped in a large, 

 rnou cellulose mantle, attainan independent existence. It passes 

 out of the parental brood-sac into the cloaca of the colony, and thence 

 to the exterior. The youngest free colonics of Pyrosoma are only 

 found at considerable depths (Chun), but older and larger stocks are 

 met with at the surface of the water. 



I L94 Two stages in the development of the colony of Pyrosoma (after Kowa- 

 levsky) In .4, the yolk-mass (do) is partly surrounded by the Cyathozooid (c), 

 while in /;. it lies entirely within the body-cavity of the latter, c, Cyathozooid; 

 d atrial pore of the Cyathozooid; d, alimentary canal ol the Cyathozooid; do 

 volk; ec, ectoderm; el, elaeoblast; en, endostyle oi the Ascidiozooid ; fl, ciliated 

 pit ■ g ganglion of the Cyathozooid ; i, inhalent orifice of the Ascidiozooid ; hs, gUl- 

 slits :' J body-cavity of the Cyathozooid ; m, cellulose mantle; n, nervous system o 

 the Ascidiozooid; p, peribranchial cavitj ; sn, lateral uerve ; v, eutodermal canal 

 uecting the Ascidiozooids with one another ; s, remains of the cell-zone. 



The organ-rudiments of the young Ascidiozooid chain are. originally, 

 direct continuations of the imperfectly developed organ in the Cyatho- 

 zooid (Figs. 192 A', L93). The ectoderm of the chain is in continuity 

 with that of the germ-disc. The intestine of the Cyathozooid is 

 continued into the enteric rudiment of the Ascidiozooid. and the 

 endostyle-fold also proceeds direct from the rudiment of this fold in 

 the germ-disc (Figs. 192, 193, en). In a similar way the peribranchial 



