38 



BRYOZOA ECTOPROCTA. 



already mentioned (p. 15), have undergone degeneration, the polypide 

 having therefore to be regenerated. In the forms mentioned above 

 the regeneration of the polypide has repeatedly been investigated; 

 notably by Nitsche (Nos. 23 and 52), Repiachoff (No. 30), Joliet 

 (No. 17), Haddon (No. 12), Ostroumoff (No. 26), and more recently 

 by Seeliger (No. 37a), Davenport (Nos. 11 and 46a), Braem (No. 

 45a), Oka (No. 52a), and Kraepelin (No. 50). * The way in which 

 the polypide develops in the statoblasts has been specially described 

 by Braem (No. 45a) and Oka (No. 52a). 



'7-^< 



Fig. 19.— Two ontogenetic stages of Plumatella fungosa (after Nitsche). A, an advanced 

 embryo. B, larva with two polypides in the act of attaching itself; the mantle-fold (J) is 

 already reflected, the apertures of evagination have moved far apart and the polypides are 

 retracted, p, the two first formed polypides ; k, k', buds of future polypides ; /, mantle-fold. 



We have already seen the rudiment of the polypide arising in the 

 primary zooecium of Bugula (Fig. 12, B, p. 29) in the form of a 

 double-walled sac, the inner cell-layer of which is said to be derived 

 through invagination from the retractile disc (see, however, the 

 statements of Prouho, p. 24), while the outer layer has probably 

 arisen from the cells of the central tissue. In the same way the 

 first rudiment of the polypides in the buds (/.-, k', Fig. 19), in the 

 regenerated individuals, and in the statoblasts are bilaminar sacs, 

 which arise through invagination at one point of the bilaminar 

 body-wall (endocyst). In the marine Bryozoa and in Plumatella, such 

 an invagination lias, from the first, a lumen which passes direct 



* Cf. Harmeii, Qiiart. Journ. Micro. Sci., Vol. xxxiii., p. 123. 



