AND DEVELOPMENT OF PYROSOMA. 233 



dicular lip x^tli of an inch deep. The aperture leads into a wide cavity about as deep 

 as the lip (y), into which the prolonged tongue of the test projects. The canal which 

 traverses the centre of this tongue, and which consequently must place the cavity of the 

 cyathozooid in communication with the exterior, appears very distinct. The appendix {h) 

 has the form of a curved tube, with its concavity turned towards the cavity of the cyatho- 

 zooid. Its anterior end is slightly enlarged, while its posterior end, also a little dilated, 

 is seated upon a slight prominence : both ends seem to be open. 



On one side of this appendix, a canal (0), xi oth of an inch long by 93-0 tli wide, passes 

 obliquely towards the cavity of the cyathozooid and apparently opens into it. Posteriorly 

 it is continued, at an obtuse angle, into a similar tube having about the same length, and 

 eventually passing into the first isthmus, now yieth of an inch long. It will be observed 

 that, notwithstanding the advanced condition of the ascidiozooids in this foBtus, their 

 upper extremities, do not rise so high as the level of the middle of the ellipsoid formed 

 by the cyathozooid and ovisac. The point at which their atrial apertures will eventually 

 be formed, consequently, can hardly be so high as the lower end of that ellipsoid. 



As has been akeady hinted, with the advance of the foetuses in size all their relations 

 become changed. The ascidiozooids, instead of presenting a fraction of the length of the 

 combined ellipsoidal cyathozooid and ovisac, and occupying only a small portion of the mass 

 of the foetal spheroid, gradually become fully thrice as high as the ellipsoid in question, and 

 form by far the greater proportion of the mass of the spheroid (fig. 15). The ovisac and 

 cyathozooid, again, diminish, not only relatively but absolutely (fig. 16), inasmuch as their 

 largest diameter does not eventually amount to more than Tijth or gV^h of an inch, while 

 the lip and the internal cavity of the cyathozooid become less distinct structures than 

 before. 



But the most curious change is that which has taken place in the test in the vicinity of 

 the cyathozooid. It has, as it were, separated itself from the latter, following the asci- 

 diozooids as their vertical diameter increases, whereby the central tongue of the test is 

 pulled completely out of the mouth of the cyathozooid, as one might pull a finger out of a 

 glove (compare figs. 18 and 19, Plate XXXL). As a consequence of this operation a 

 cavity, which gradually increases in dimensions, is developed between the outer surface 

 of the cyathozooid and the inner wall of the test ; and as the atrial ends of the ascidiozooids 

 ascend in consequence of the growth of the latter, they open into this cavity, Avhich thus 

 manifests itself as the cloaca (fig. 19). The tongue-like prolongation of the test becoming 

 pulled out and flattened as the cloaca widens, ultimately ceases to project into the cavity 

 of the latter, and becomes converted into the lip of its aperture. In fig. 19 it still 

 protrudes for some distance into the cloacal cavity. 



2. The Ascidiozooids. — Prom their small size, flattened form and general opacity, it is 

 by no means so easy to trace satisfactorily the successive changes by which the other seg- 

 ments of the blastoderm are converted into perfect ascidiozooids, as it is to follow out the 

 development of the buds. Nevertheless, knowdng the latter process, it is not difficult to 

 interpret the appearances presented by the segments of the blastoderm, in the course of 

 their development. 



When the blastoderm first becomes marked out into those segments which eventually 



VOL. XXIII. 2 I 



