RHABDOPLEUEA AND CEPHALODISCUS. 19 



or collar region would have the effect of removing the central nerve mass and the 

 collar pores from the anal region. Both collar canals and proboscis canals are regarded 

 by Schepotieff as " nephridia " (1905, p. 801). 



The trunk coelom does not open to the exterior. It is continued into the stalk, 

 as was first noticed by Lankester, and it is divided by a median septum, which runs 

 into the stalk, dividing it into right and left parts more completely than does the 

 corresponding septum in Cephalodiscus. The longitudinal muscles of the stalk, says 

 Harmer (10, p. 78) are related to those of the body exactly as in Cephalodiscus, and 

 there are two blood-vessels. The posterior of these, according to Fowler (1904), is 

 continuous with the lining of the alimentary canal; but Harmer is inclined to doubt 

 this, and to regard it as a true blood-vessel, homologous with the posterior blood- 

 vessel of the stalk of Cephalodiscus. There is a nerve tract on the anterior or ventral 

 side of the stalk, exactly as in Cephalodiscus (Fowler, 1904). 



The gonads of Rhabdopleura are not well known, owing to the fact that most 

 of the material examined has been collected at the wrong time of the year. The 

 only definite information on the subject is that given by Lankester (13) and 

 Schepotieff (1905, p. 801), according to whom the testis is unpaired, and situated 

 on the right side. It is a long sac, which extends from the posterior end of the 

 visceral mass to the anal papilla, and the hinder part is sometimes swollen, and 

 contains developing spermatozoa, whereas the rest of the sac is filled with ripe 

 spermatozoa. The ovary of Rhabdopleura may be said to be unknown, since the 

 account of the ovary and testis as occurring in different parts of the peduncle, 

 given by Conte and Vaney (C. R. Acad. Sci., 1902, p. 64) is too brief to be intelligible.* 



In describing the growth of the bud of Rhabdopleura Schepotieff notices (1905, 

 p. 803) that the coelom is divided into six parts at an early stage, two in the 

 shield, two in the collar, and two in the trunk, the last continuous for some time 

 with that of the parent stolon. The right half of the shield cavity is much smaller 

 than the left ; it becomes median, and after undergoing further reduction in size, 

 persists as the pericardium. The gut develops as a solid mass of cells, subsequently 

 hollowed. The notochord develops from the anterior part of this, and the stomodaeum 

 opens into the middle part. The proctodaeum is a solid mass of ectodermal cells, 

 hollowing later, and becoming connected with the hind end of the primitive gut. 

 Fowler, it is to be noted (1904, p. 28), regards the notochord as developed from 

 the stomodaeum, and not from the primitive gut. 



In reviewing the above remarks on the polypides of Rhabdopleura, it will be 

 seen that in bodily structure Rhabdopleura agrees very closely with Cephalodiscus, 

 the only important differences being in the position of the collar pores, the number 

 of the plumes, and the absence of pharyngeal perforations or gill-slits. The 



* These authors are also responsible for a denial of the existence of collar pores, and divisions of the 

 coelom, and the presence of a notochord, the structure recognised as notochord by Fowler (1892) being, according 

 to them, nothing more than the anterior end of the peduncle. Fowler's observations, it is well to bear in mind, 

 have since been amply confirmed by Schepotieff (1904 and 1905). 



R 2 



