CCELENTERATA : CTENOPHORA. 179 



spond in form, in relative size, and mode of communication 

 with the somatic cavity. The funnel and apical canals of 

 Pleurobrachia, though more distinctly marked out, are the 

 homologues of those parts of the general cavity, which in 

 Actinia are central in position, and underlie the free end of the 

 digestive sac. So also the paragastric and radial canals may 

 be likened to those lateral portions of the somatic cavity of 

 Actinia which are not included between the mesenteries. 

 Lastly, the ctenophoral canals of Pleurobrachia and the somatic 

 chambers of Actinia appear to be truly homologous, the chief 

 difference between the two forms being, that while in the latter 

 the body-chambers are wide and separated by very thin parti- 

 tions, they are in Pleurobrachia reduced to the condition of 

 tubes ; the mesenteries which intervene becoming very thick 

 and gelatinous, so as to constitute, indeed, the principal bulk 

 of the body." The "apical" canals, again, by which the di- 

 gestive sac communicates inferiorly with the external medium, 

 may be compared with the perforation which is found in some 

 of the Actinidcz (Cerianthus and Peachia] traversing the axis of 

 the base or foot. 



The remaining members of the Ctenophora conform in most 

 essential respects with Pleurobrachia, the most important dif- 

 ferences being found in the canal -system. For purposes of 

 comparison this system may be divided into four portions as 

 follows: i. The "axial system," consisting of the mouth, 

 stomach, funnel, and apical canals j 2. The "paraxial system/' 

 comprising the paragastric canals ; 3. The * radial system," 

 comprising the primary, secondary, and tertiary radial canals ; 

 4. The "ctenophoral system," consisting of the tubes which 

 run underneath the locomotive bands. 



In JBeroe, which is in other respects very similar to Pleuro- 

 brachia, the axial system of canals is the same as we have seen 

 in the latter. The paraxial system, however, consists of two 

 pairs of paragastric canals, which, instead of terminating cae- 

 cally, open into a circular canal which surrounds the mouth. 

 The ctenophoral canals, likewise, open into the oral vessel, in- 

 stead of terminating csecally as in Pleurobrachia. Lastly, the 

 radial system is not developed, the ctenophoral canals simply 

 curving round towards their apical extremities, and opening 

 into the funnel directly. 



Amongst the Beroidce the mouth extends entirely across the 

 oral extremity of the body; hence they have been termed 

 Eurystomata, the term Stenostomata being applied collectively 

 to all the other Ctenophora. 



The Beroidcs further differ from Pleurobrachia in being desti- 



