30 BRYOZOA ECTOPROCTA. 



more fully discussed in a special section on the further development 

 of the rudiment of the polypide (pp. 43, 44, and 55). 



In considering the metamorphosis of the Biujula larva, which 

 is identical with that of the types hitherto described, we are at 

 once struck by the fact that the larval intestine degenerates in all 

 cases, and that the intestine of the primary individual is formed 

 anew from an independent rudiment on the aboral surface. This 

 fact, however, is explained by the capacity possessed by the Bryozoa 

 in certain cases of degenerating and again regenerating the alimentary 

 canal or even the whole polypide (see p. 55). We are therefore not 

 obliged to regard the polypide of the primary individual as a second 

 generation of the Bryozoan colony, derived by budding from the 

 larva. The production of the primary individual from the larva 

 rather comes under the category of metamorphosis, although this 

 latter becomes very complicated in consequence of the far-reaching 

 degeneration of the larval organs, and on account of the incon- 

 spicuous form assumed by the rudiments of the future parts of 

 the permanent body in the larva, these rudiments being somewhat 

 comparable to the imaginal discs of the Insecta. 



With regard to these rudiments, that of the polypide is repre- 

 sented in the larva by the retractile disc. The rudiment of the 

 endocyst of the primary zooecium lies partly in the mantle-cavity 

 and partly in the sucker-invagination. The body-wall of the primary 

 individual is thus present in the larva in an invaginated condition, 

 so as to afford the corona, as the locomotory organ of the larva, more 

 room for development. 



While most of the parts of the primary zooecium are thus waiting in the larva 

 in an invaginated and apparently functionless condition, it is a striking fact 

 that the retractile disc, which has hitherto been regarded as the rudiment of 

 the polypide, seems to be of functional importance to the larva (sensory 

 organ ?). On this account, the observation of Phouho recorded above 

 (j). 24) seems significant, according to which the retractile disc does not pass 

 directly over into the rudiment of the polypide, but after its invagination 

 undergoes a degeneration similar to that of the other larval organs, while, at 

 the place formerly occupied by it, a new two-layered rudiment develops, viz., 

 that of the polypide. 



The bending over of the corona and the formation of the vestibule, in the 

 wall of which are included the coronal cells, the pyriform organ, and a part 

 of the sucker, there to undergo degeneration, will appear less remarkable if we 

 consider how often larval parts which have become useless, instead of being 

 thrown out, sink into the interior of the individual undergoing metamorphosis, 

 there, after degenerating, to be utilised as food-material. Under this aspect, 

 these processes appear comparable to the retrogression of the embryonic mem- 

 branes and the formation of the so-called dorsal organ in Insect embryos. 



