PYKOSOMA 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). 

 < >nly now (Fig. 196 B) does the colony, which is enveloped in a large, 

 common cellulose rmmtle, attain an independent existence. It passes 

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

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

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

 met with at the surface of the water. 



Kh.. 1 1*4. -Two stage* in the development of the colony of J'i/r<ixi>iii.n i;ii'ter KOWA- 

 I.KV.SKY). In .1, the yolk-mass (do) is partly surrounded by the Cyathozooid (<). 

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

 . i trial pore of the Cyathozooid; d, alimentary canal of the Cyathozooid; do, 

 yolk; r<\ ectoderm; el, elaeoblast ; en, endostyle of the Ascidiozooid ; ,/?, ciliated 

 pit ; //. ganglion of the Cyathozooid ; i, inhalent orifice of the Ascidiozooid ; ks, gill- 

 slits ; /. body-cavity of the Cyathozooid ; m, cellulose mantle; /i, nervous system of 

 the Ascidio/ooid ; //, peribranchial cavity; sn, lateral nerve; v, entoderinal canal 

 connecting the Ascidiozooids with one another ; z, 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 7>, 193). 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 



