560 



POLYZOA. 



distribute the species, the larva comes to rest and attaches itself to 

 some foreign object. The attachment is effected by the ventral 

 adhesive organ, which is evaginated and applied to the substratum 

 (Fig. 449). At the same time the larval organs are in process of 

 disappearance. The alimentary canal, if present in the larva, totally 

 disappears, and is in no way concerned in the formation of the 

 digestive tube of the adult. The pyriform organ and the velum 

 share in the general fate, and the larva becomes reduced to a layer 

 of epithelium surrounding a central mass of cells and broken-down 

 larval organs. 



The attached organism is now called the primary zooecium, and 

 the degenerated larval organs in its interior constitute the first brown 



FIG. 450. Two stages of the primary zooecium, showing the development of the first polypidf 

 of Bugula mlatlius (after Vigelius, from Korschelt and Heider). A, imagination of ectoderm 

 for the formation of the vesicular rudiment of the first polypide ; b cells of the mesoderm 

 of the future polypide ; e ectoderm. B, somewhat older stage, showing the two-layered 

 vesicular rudiment of the first polypide; a ectoderm lining the vesicle; 6 mesoderm. 

 In A the internal mass resulting from the degenerated larval organs is omitted. 



body. The next stage consists in the invagination inward of the 

 aboral ciliated disc* (Fig. 450 A, a) ; the pit so formed becomes 

 converted into a vesicle (Fig. 450 B), round the walls of which 

 some of the internal cells arrange themselves. These latter give 

 rise to the mesoderm of the future polypide, while from the lining 

 cells of the vesicle, which are ectoderrnal in origin, are developed 



* According to Prouho the aboral disc, after invagination, shares the fate of 

 the other larval organs, and the polypide rudiment is derived from an invagina- 

 tion of ectoderm of the primary zooecium, or it may be a new formation possibly 

 from the internal tissue. 



