308 



POLYZOA. 



Chaetopod becomes very much strengthened by taking as types 

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



FIG. 134. Two STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metsch- 

 nikoff.) 



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

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



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 Chaetopods, Rotifera, etc., which become fixed in tJu: 

 adult by the extremity of tlieir prce-oral lobe. 



The attachment of the larva by the prae-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 tJie 

 pJtenomenon of alternations 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 diagramma- 

 tically put in black for greater distinctness. 



