ASCIDIACEA DEVELOPMENT OF THE FREE-SWIMMING LARVA. 355 



BALFOUR has pointed out that, since a similar attaching apparatus is found 

 in Amphibian larvae and (in front of the mouth) in the larvae of many 

 Ganoids (Acipcnser, LepidoKteus) we may perhaps have here an inherited 

 feature common to the Chordata. It seems doubtful, however, to what ex- 

 tent these structures are really homologous and not merely analogous. 



After the egg-envelope has burst, the larva straightens out. The 

 tail then forms a direct posterior continuation of the longitudinal 

 axis of the body (Figs. 167 and 173 A, p. 375). 



FIG. 166. Transverse section through an attached larva of Phallusia tnammillata 

 (after KOWALBVBKY). a, mesenchyme-cells in the act of passing through the 

 ectoderm ; b, mesenchyme-cells in the cellulose mantle ; d, alimentary canal ; 

 ec, ectoderm ; ms, mesenchyme-cells ; <>t, otolith ; s, transverse section through 

 the sensory vesicle ; t, cellulose mantle. 



The mantle. The ectoderm-cells, which originally were somewhat 

 cubical but assumed a more flattened form, at the time when the 

 caudal region develops, secrete, at their outer surface, a homogeneous 

 cuticular layer which, from its first appearance, gives a cellulose 

 reaction. This is the first rudiment of the Ascidian test or tunic. 

 In the caudal region, this layer grows out to form a median dorsal 

 and a ventral fin (Fig. 169, Jl, p. 363). While, in Doliolum and 

 Appendicularia, such a simple, homogeneous cuticular layer is 

 retained throughout life, in the Ascidiacea and Hemimyaria it is 

 considerably thickened, single cells immigrating into the cellulose 



