PLUMES OF CEPHALODISCUS. 247 



the connection being but a mere hollow ridge. The ridge is 

 certainly connected with the hinder edge of the collar (text- 

 fig. 10, 2^0.1.), and. the middle part of each lateral flap with 

 the middle of the length of the collar, i.e. not with the front 

 edge nor the hind edge ; in the region of the collar canal, 

 which is set on the postero-dorsal edge of the collar, about 

 midway between the dorsal and ventral surfaces, the base of 

 the flap is near the front edge. But the relations of the 

 plumes to the post-oral lamella can be clearly understood in 

 spite of this; and it is a very significant circumstance, in 

 connection with the view that the post-oral lamella and the 

 plumes belong to one and the same system, that in the 

 development of the buds of Cephalodiscus each new 

 plume arises between the last-formed plume and the end of 

 the post-oral lamella; one might almost say that each new 

 plume is differentiated from that part of the post-oral lamella 

 immediately in contact with the last-formed plume. The 

 post-oral lamella may thus be regarded as composed of 

 postero-ventral plumes which fail to differentiate as separate 

 plumes. 



In the adult the food-grooves of the axes of the first and 

 second pairs of plumes face ventrally,^ i.e. towards the dorsal 

 wall of the front part of the shield ; the grooves of the 

 third and fourth pairs face laterally, and those of the fifth 

 and sixth pairs dorsally or dorso-laterally. Tracing these 

 grooves basally one finds that those of the right-hand plumes 

 bend round and converge to the right side of the mouth, and 

 those of the plumes of the left side of the body to the left 

 side of the mouth, the food current being in all probability 



' It is to be noted that Haimer orientates the polypide of Cephalodiscus 

 in sucli a way that the antero-posterior axis of the body is a line passing from 

 the middle of the buccal shield through the central nerve mass, and ending 

 on the rectal side of the body a little below the anus, so that the visceral 

 mass is regarded as a ventral downgrowth (' Pterobranchia of the " Siboga " 

 Expedition,' 1905, p. 23). In the present paper the ordinary orientation is 

 adopted, namely, plume-apices anterior, stolon and face of shield ventral, 

 intestine dorsal, rounded end of visceral mass posterior. 



VOL. 51, PART 2. — NEW SERIES. 19 



