374 TUNICATA. 



arranged into a simple strand. The degeneration of the chorda-cells 

 which are distinguished by their granular contents, is finally brought 

 about by phagocytosis. This is also the case with the muscle-cells. 

 The ectoderm- cells, as the caudal section shortens, become larger 

 (Fig. 172 A), and spherical, strongly refractive bodies appear in 

 them, so that they come to resemble the so-called granulated spheres 

 of the pupa of the 'Musddae (Vol. iii., p. 379). When the internal 

 organs of the caudal section have been completely taken up into 

 the body-cavity of the trunk, the ectoderm is invaginated (Fig. 172 

 B, ens). This invagination soon becomes completely abstricted from 

 the epidermis of the larva and then forms a closed vesicle lying 

 within the body-cavity ; -the cells of this vesicle soon lose their 

 cohesion, the lumen disappears, and finally nothing remains but 

 a mass of detached and gradually disintegrating granular cells. The 

 gelatinous envelope (Fig. 173 C, ss) of the caudal region is, finally, 

 lost either by being simply absorbed according to Kupffer's obser- 

 vations, or thrown off, as Seeligek and Milne-Edwards agree in 

 maintaining. 



Since the attachment of the larva is accomplished by means of the 

 anterior end of the body, the oral aperture (inhalent orifice) appears 

 to lie near the point of attachment (Fig. 173 B). In the adult 

 Ascidian, on the contrary, the oral aperture lies at the end of the 

 principal axis of the body opposite to the point of attachment (Fig. 

 173 C). This shifting of the position of the oral aperture is the 

 result of a rotation made by the body round its transverse axis 

 after attachment, during which the part of the body between the 

 mouth and the point of attachment lengthens. This lengthening, in 

 Clavelina, according to Seeliger, is made possible by the develop- 

 ment of a deep infolding of the surface of the body (Figs. 170, 173 

 A and B, /) which slightly separates a pre-oral portion carrying the 

 adhering papillae from the rest of the body. This pre-oral region 

 represents the basal section of the young Clavelina from which the 

 branching stolon soon grows out. The folding just mentioned 

 renders it possible for the Clavelina, which originally was placed 

 with its longitudinal axis at right angles to the basal plane (surface 

 of attachment), first to bend sharply towards this plane and then 

 to lie with its longitudinal axis parallel to it, finally however, to 

 rise up from it in such a way that the oral aperture comes to 

 lie opposite to the point of attachment. During this rotation round 

 the transverse axis, which was pointed out first by Kupffer and 

 later by Seeliger, the angle passed through is one of almost 180.° 



