RIIABDOPLEURA. 579 



The relation of the polypide to its dwelling in Rhab- 

 dopleura is totally unlike that to which we are accus- 

 tomed in the ordinary, typical polyzoon. There is no 

 endocyst, or at most only the remains of one, and con- 

 sequently no tentacular sheath ; and the polypide is 

 therefore wholly unconnected with its cell. With the 

 endocyst or enveloping sac go the perigastric cavity 

 and the system of muscles by which the polypide is 

 moved up and down within it. Its only connexion is 

 with a cylindrical chitinous rod, inclosing a soft, cellular 

 core, and traversing the whole of the adherent portion 

 of the zoarium, to which it is attached by means of a 

 contractile cord. This cord is united to the body of the 

 polypide, not at the bottom of the stomach, but some way 

 up on one side ; it possesses a high degree of contrac- 

 tility ; and it is by means of it that the retraction of the 

 polypide is effected. It is probably homologous with the 

 " funiculus " of the ordinary polyzoon, whilst in the soft 

 core of the axial rod (which, according to Sars, resembles 

 in structure the contractile cord) we may recognize the 

 equivalent of the endosarc as it appears (for instance) in 

 the stems of the Ctenostomata. The septate condition of 

 the adherent portion of the coenoecium has also its parallel 

 in the latter. 



The polypide, as we might expect, is sluggish in its 

 movements, both in issuing from its cell and retreating, 

 and has none of the activity of its tribe. 



The annulated condition of the chitinous coenoecium is 

 also a peculiarity which does not occur in the other known 

 Polyzoa. In the general character of the zoarium, as well 

 as in several other important points, the present form 

 makes an approach to the freshwater group of the Phy- 

 /itcloltemata. 



