524 THE ANATOMY OF INVERTEBRATED ANIMALS. 



VII. A foetus with the blastoderm divided into five seamen ts, of whi(th the cyatho- 

 zooid ([.) is the largest. VIII. A foetus, tlie ascidii)Zooids of which half encircle 

 the base of the cyaUioz ;oid. IX. Fcetu<, the most advanced stage observed. The 

 remains of tlie coujoiued cyathuzooid and ovisac are hidden by the circle of 

 ascidiozoSids. ^ , . , 



The letters have the same signification in all the figures, a, test; a^, labial process; 

 a*, lip of the cloacal apukure; a^. ceils of the embryonic test; e, oral aperture; 

 «■-' atrial aperture ; i, eudostyle; l'^, P, branchial sac and siiirmata; r*, lieart; r^, 

 stolons of the adult ascidiarium ; 7-*, stolons of the embryonic ascidiarium ; .«, ovi- 

 sac; f, testis; w, «', ovum; w', peduncle of a bud; x, the alimentary portion of 

 the endoderm entering into a bud; x% its genei-ative portion; a;-, tae ectoderm 

 entering into a bud ; «■, the oeleoblast ; z, ganglion. 



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

 ' zooids. B, mouth of the cyathozoSid. 



which develop new ascidiozooids, are j:^iven off at intervals ; 

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

 cidiozooids retain no permanent connection with one another. 

 In the BotryUidm^ the zooids are arranp^ed 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 Bjtryllm, the process of budding is liighly 

 instructive, as it exemplifies the manner in which gemmation 

 occurs in the Ttmicata in general.^ 



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 e.xterior surface, and their atrial apertures into 

 the interior of t'l^e cylinder. The hsemal aspect of each zooid 

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

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

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



E7ery zo'Jid multiplies by gemmition 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 

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

 elevation, which projects into the surrounding substance of 

 the test, and contains a m iss of mesoblastic cells, one of which 



1 Huxlev, " Anatorriv and Dgvelopment of Pt/rosoma.''^ (" Trans. Linutean 

 Society," 18G0.) Kowulevvsky Cl. c, infra, p. 610). 



