524 THE ANATOMY OF INVERTEBRATED ANIMALS. 



VII. A foetus with the blastoderm divided into five segments, of which the cyatho- 

 zoOid (I.) is the largest. VIII. A foetus, the ascidiozoOids of which half encircle 

 the base of the cyathozoOid. IX. Foetus, the most advanced stage observed. The 

 remains of the conjoined cyathozo5id and ovisac are hidden by the circle of 

 ascidiozoOids. 



The letters have the name signification in all the figures, <z, test; a 3 , labial process; 

 a 4 , lip of the cloacal aperture ; a 5 , cells of the embryonic test ; e, oral aperture ; 

 p*, atrial aperture ; i, endostyle ; J 2 , / 3 , branchial sac and stigmata ,; r, heart; r 2 , 

 ptolons of the adult ascidiarium ; r 4 , stolons of the embryonic ascidiarium ; , ovi- 

 sac; t, testis ; u, u', ovum; ;', peduncle of a bud; x, the alimentary portion of 

 the endoderm entering into a bud; a? 1 , its generative portion; 2 , the ectoderm 

 entering into a bud ; , the celeoblast ; z, ganglion. 



I., II., III., IV., V. Segments of the blastoderm. I. CyathozoOid. IV.-V. Ascidio- 

 zooide. , mouth of the cyathozoOid. 



which develop new ascidiozooids, are given off at intervals ; 

 but, more commonly, the ascidiarium is massive, and the as- 

 cidiozoSids retain no permanent connection with one another. 

 In the BotfyUidCB, the zooids are arranged in whorls around 

 a common central cavity, or cloaca, into which the atria of 

 all the members of the whorl open. In Pyrosoma, which is 

 a sort of floating Botryllus, the process of budding is highly 

 instructive, as it exemplifies the manner in which gemmation 

 occurs in the Tanicata in general. 1 



The ascidiarium of Pyrosoma (Fig. 150, I.) has the form 

 of a hollow cylinder, rounded and closed at one end, truncated 

 and open at the other, formed of a firm transparent test, in 

 which the zooids are arranged in whorls. Their oral apertures 

 open on the exterior surface, and their atrial apertures into 

 the interior of the cylinder. The hasmal aspect of each zooid 

 is turned toward the closed end of the cylinder. The bran- 

 chial sac has the ordinary structure, and each zooid is provid- 

 ed with a testis and with an ovisac, containing a single ovum. 



Every zooid multiplies by gemination from a region of the 

 body which lies immediately behind the extremity of the en- 

 dostyle. Close to the heart, attached to a short csecal process 

 of the endoderm which constitutes the extremity of the endo- 

 style, and which I have termed the endostylic cone, is a cellu- 

 lar mass the remains of that mass of indifferent tissue which 

 I have called the generative blastema, and from which the gen- 

 erative organs of the gemmiparous zooid have been developed 

 (Fig. 150, II.). The endostylic cone elongates, and, curving 

 toward the haemal side of the body, applies itself closely to the 

 ecoderrn (Fig. 150, III.). The latter grows out into a conical 

 elevation, which projects into the surrounding substance of 

 the test, and contains a mass of mesoblastic cells, one of which 



1 Huxley, " Anatomy and Development of Pyrosoma." (" Trans. Linnsean 

 Society," 1860.) Kowalewsky (1. c., infra, p. 616). 



