266 THE PROTOCHORDATA. 



The collar-coelom is continued posteriorly into the opercu- 

 lum, and anteriorly into the twelve tentacular appendages. 



Finally, behind the collar comes the region of the body 

 containing the viscera, which are surrounded by the third 

 section of the coelom. 



Only the female reproductive organs have been observed 

 up to the present time in Cephalodiscus. They occur as 

 a pair of gonadic sacs, opening to the exterior on each 

 side of the dorsal middle line between the anus and the 

 central nervous system. The latter is very simple, being 

 represented merely by a dorsal thickening of the ectoderm, 

 with nerve-fibres in the region of the collar and posterior 

 portion of proboscis. 



Finally, a short notochordal diverticulum projects into 

 the base of the buccal shield as in Balanoglossus. 



Rhabdopleura differs considerably from Cephalodiscus 

 in many respects, but, nevertheless, has some fundamen- 

 tal characteristics in common with it. In Rhabdopleura 

 the individuals of a colony are not independent, but are 

 connected with each other by a common cord or caulns, 

 which represents the remains of the contractile stalks of 

 the polyps. As the growth of the colony proceeds, the 

 distal portions of the stalks (i.e. the portions farthest away 

 from the animals) become shrunken and hard. The buds 

 arise from the soft portions of the caulus, and never be- 

 come detached as they do in the case of Cephalodiscus. 

 There is only a single pair of tentacular plumes in Rhab- 

 dopleura. 



FOWLER has recently shown that in Rhabdopleura the 

 coelom, whose existence was first established by LAN- 

 KESTER, exhibits the same subdivisions as have been 

 mentioned above for Cephalodiscus; namely, (i) the cavity 

 of the large buccal shield, (2) the collar-cavity opening 



