CCELENTERATA I ACTINOZOA. I 45 



and generally admitted to be a rudimentary form of nervous 

 system. The reproductive organs of Pleurobrachia are in the 

 form of folds, containing either ova or spermatozoa, and 

 situated beneath the endodermal lining of the ctenophoral 

 canals, one on each side. 



The embryo Pleurobrachia is at first rudely cylindrical in 

 form, a belt of cilia passing round the middle of its body. 

 This soon breaks up into two lateral groups, which eventually 

 disappear altogether, " the ctenophores, at first very broad and 

 few in number, at an early period taking on the performance 

 of their special function " (Greene). 



As regards thehomologies between Actinia and Pleurobrachia, 

 the following may be quoted from Professor Greene : 



" If now a comparison be made between this nutrient 

 system " (the canal-system of the Ctenophora} " and that of 

 Actinia, the digestive sacs of the two organisms are clearly 

 seen to correspond 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 mesen- 

 teries. 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 partitions, they are in Pleurobrachia reduced to the con- 

 dition 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 digestive sac communicates ififeriorly with the 

 external medium, may be compared with the perforation which 

 is found in some of the Actinidce, (Cerianthus and Peachid] 

 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. 2. The "paraxial system/' 

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

 comprising the primary, secondary, and tertiary radial canals. 



K 



