132 



MANUAL OF ZOOLOGY. 



and then again into two, as they proceed towards the peri- 

 phery of the body. Thus, the two "primary" radial canals 

 produce four " secondary" canals {k), and these, in turn, give 

 rise to eight " tertiary" radial canals (/), which finally termi- 

 nate by opening " at right angles into an equal number of 

 longitudinal vessels, the ' ctenophoral canals' (/), whose course 

 coincides with that of the eight locomotive 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 circulation of the included 

 nutrient fluids is thus maintained. 



Immediately within the apical pole is situated a small cyst 

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



^'E- 37'— Morphology of Ctenophora. i. Diagrammatic transverse section ot Pleu- 

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

 canals : / / Tertiary radial canals ; g Tentacle. 



2. Longitudinal ^i^ci\oaoi Pieurobrachia. a Mouth; ^ Digestive cavity ; ^ Fun- 

 nel ; d d Paragastric canals ; e e Apical canals ; y Ctenophoral canal ; g Tentacle ; 

 h Ctenocyst. (After Greene.) 



"ctenocyst" {K). In structure the "ctenocyst" consists of a 

 spherical vesicle, lined with a ciliated epithelium, and filled 

 with a clear fluid, which contains mineral particles, probably 

 of carbonate of lime. Resting upon the ctenocyst is a small 

 ganglionic mass, giving origin to a number of delicate fila- 

 ments, and generally admitted to be a rudimentary form of 

 nervous system. The reproductive organs of Pkurobrcuhia 

 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 



