MANUAL OF ZOOLOGY. 



bands. These canals end caecally both at their oral and apical 

 extremities" (Greene). The whole of this complex canal- 

 system is lined by a ciliated endoderm, and a constant circula- 

 tion of the included nutrient fluid is thus maintained. 



Fig. 90. Morphology of Ctenophora. i. Diagrammatic transverse section of Pleitrc- 

 brachia. b Digestive cavity ; i i Primary radial canals ; k k Secondary radial canals ; 

 / / Tertiary radial canals ; g Tentacle. 



2. Longitudinal section of P leurobrachia. a Mouth ; b Digestive cavity ; c Fun- 

 nel ; d d Paragastric canals ; e e Apical canals ; f Ctenophoral canals ; g Tentacle ; 

 h Ctenocyst. (After Greene.) 



Immediately within the apical pole is situated a small cyst 

 or vesicle, supposed to be an organ of sense, and termed the 

 " ctenocyst " (h). In structure the "ctenocyst" consists of a 

 spherical vesicle, lined with a ciliated epithelium, and rilled 

 with a clear fluid, which contains mineral particles, probably 

 of carbonate of lime. Just beneath the ctenocyst is a cellular 

 mass, which has been described as giving off eight filaments 

 running along the ctenophores, and is generally believed to be 

 a nervous system. Eimer denies that the central ganglionic 

 mass is nervous, but describes a plexiform nervous system. 

 The reproductive organs of Pleurobrachia are in the form of 

 folds, containing either ova or spermatozoa, and situated be- 

 neath 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 primitive ctenophores are four in 

 number, each ultimately breaking up into two. 



As regards the homologies 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 corre- 



