ASCIDIACEA FIXATION AND METAMORPHOSIS. 



373 



(Fig. 173 ft), to which they are then appended as a short truncated 

 process. The internal organs of the caudal region are retracted more 

 and more into the trunk (Fig. 172). They there coil up spirally, but 

 the separate elements (the chorda, the muscle-bands, the neural tube) 

 which compose this coiled strand retain for a long time their relative 



A 



FIG. 172. Degeneration of the caudal region during metamorphosis in the larva of 

 Pliallusm mammillata (after KOWALEVSKY). A, longitudinal section through an 

 early stage ; B, posterior portion of a longitudinal section through an older stage. 

 ch, chorda-cells ; ec, ectoderm ; en.fi, invaginated epidermis of the tail ; ms, mesen- 

 chyine-cells in the act of passing through the ectoderm ; mt, cellulose mantle ; mz, 

 muscle-cells of the tail ; >i, nerve-cells of the caudal section ; ns, neural tube. 



positions (c/. Fig. 172 B, x, where the strand is cut through trans- 

 versely). The degeneration of the chorda commences, according to 

 KOWALEVSKY'S most recent researches (No. 32), by the disappearance 

 of its gelatinous substance, while its cells (Fig. 172, ch) again become 



