440 



POLYZOA. 



pair of invaginated respiratory appendages of the vagus region. The genital 

 organs and ducts have approximately the same location as in many arachnids 

 and Crustacea, that is, just behind the vagus region. 



The thoracic appendages of Cephalodiscus develop in an approximately 

 regular order, from before backward, like those of arthropods. (Fig 300, C,ZX) In 

 Rhabdopleura, the arms probably represent a single pair of enlarged, antenna- 

 like cephalic appendages. (Fig. 300, A,B.) 



Attachment is effected, as in the tunicates, by a postoral haemal outgrowth 

 probably representing a special modification of the posterior part of the cephalic 

 navel. (Figs. 299, 300.) 



FIG. 300. Diagrams of Rhabdopleura. A, Adult, from neural surface; B, from side, in optical section; C, D, 

 larvae of Cephalodiscus, from the neural surface. 



It will be recalled that the cephalic navel is a center of convergent growth, and 

 that the location of the center, and the nature of the events that take place there 

 is largely dependent on the volume of the yolk sphere and the rate at which the 

 growing tissues spread over and enclose the haemal surface. The conditions 

 created at the closing area, where the advancing lines of equal growth and dif- 

 ferentiation tend to meet and annul one another, may result, at the cephalic end, 

 in an ingrowing tube, the opening of which becomes the haemastoma, and at the 

 caudal end, as in the tunicates and Pterobranchia, in a stolon-like outgrowth that 

 retains an indefinite power of growth and that becomes the seat of successive 

 generations of new buds. 



VI. THE POLYZOA. 



The polyzoa likewise may best be interpreted as descendants of primitive 

 arthropods of the cirriped type. They do not develop a clearly marked metameric 



