266 THE PRQTOCHORDATA. 



The coUar-ccelom 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 ccelom. 



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. 



Rliabdoplciira differs considerably from Cephalodiscus 

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

 tal characteristics in common with it. In Rhabdopleura 

 the individuals of a colonv are not independent, but are 

 connected with each other by a common cord or caiilus, 

 which represents the remains of the contractile stalks of 

 the polvps. As the growth of the colonv proceeds, the 

 distal portions of the stalks {i.e. the portions farthest awav 

 from the animals) become shrunken and hard. The buds 

 arise from the soft portions of the caul us, 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 

 co^lom, whose existence was first established bv Lan- 

 KESTEK, exhibits the same subdivisions as have been 

 mentioned above for Cephalodiscus ; namelv, ( i ) the cavit\- 

 of the large buccal shield, [2) the coUar-cavitv opening 



