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MANUAL OF ZOOLOGY. 



at the " apical pole." These canals are known as the " apical 

 canals " (e), and their apertures as the " apical pores." From 

 the funnel two other pairs of canals are given off. Of these, 



Fig- 35- Morphology of Ctenophora. i. Diagrammatic transverse section of Pleuro- 

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

 canals ; / / Tertiary radial canals ; g Tentacle. 



2. Longitudinal section of Pleurobrachia. a Mouth ; b Digestive cavity ; 

 e Funnel ; d d Paragastric canals ; e e Apical canals ; f Ctenophoral canal ; 

 g Tentacle ; h Ctenocyst. (After Greene.) 



one pair known as the "paragastric canals " turns upwards, 

 one running parallel to the digestive sac on each side (d\ and 

 "terminating caecally before quite reaching the oral extremity." 

 The second pair of canals (/) the so-called "radial canals" 

 branch off from the funnel laterally, each dividing into two, 

 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 

 " ctenocyst " (h). 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 Pleurobrachia 



