POLYZOA. 255 



entirely atrophies in the adult after the attachment has been effected 

 in the region of this disc. 



The comparison between the Polyzoon larva and that of a 

 Chsetopod becomes very much strengthened by taking as types 



_B 



FIG. 134. Two STAGES IN THE DEVELOPMENT OF MITEAEIA. (After Metschnikoff.) 



m. month ; an. anus ; sg. supra-oesophageal ganglion ; br. and b. provisional 

 bristles ; pr. b. pree-oral ciliated band. 



Mitraria 1 (fig. 134) and Cyphonautes (fig. 133). The similarity 

 between these two forms is so striking that I am certainly inclined 

 to view the larvae of the Polyzoa as trochospheres similar to those of 

 Chaitopods, Rotifera, etc., which become fixed in the adult by the ex- 

 tremity of their prcv-oral lobe. 



The attachment of the larva by the prse-oral lobe is not more 

 extraordinary than the attachment of a Barnacle by its head, and 

 after such a mode of attachment the atrophy of the supra-cesophageal 

 ganglion would be only natural. 



There is one important fact which deserves to be noted in the 

 development of the Polyzoa, viz. that if the suggestion in the text 

 as to the mode of development of the adult from the so-called larva 

 is accepted, the Polyzoa exhibit universally the phenomenon of alter- 

 nations of generations. The ovum gives rise to a free form which 

 never becomes sexual, but produces by budding the sexual attached 

 form. 



1 The larva of Mitraria is figured with the aboral surface turned upwards, instead of 

 downwards, as in the figure of Cyphonautes. The ciliated band is also diagramniatically 

 put in black for greater distinctness. 



