112 



MANUAL OF ZOOLOGY. 



The mouth of Pleurobrachia (fig. 30, a) opens into a fusi- 

 form digestive sac, or stomach (6), the lower part of which is 

 provided with brown cells, supposed to discharge the functions 

 of a liver. The stomach opens below into a shorter and wider 

 cavity (c), termed the * funnel,' from which two canals diverge 

 in the direction of the vertical axis of the organism, to open 

 at the ' apical pole.' These canals are known as the * apical 



9- 



Fig. 30. Morphology of Ctenophora. 1. Diagrammatic transverse section of 

 Pleurobrachia. b. Digestive cavity ; i i. Primary radial canals ; k k. " 

 Secondary radial canals ; I I. Tertiary radial canals; g. Tentacle. 



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

 c. Funnel ; d d. Paragastric canals ; e e. Apical canals ; /. Ctenophoral 

 canal ; g. Tentacle ; h. Ctenocyst. (After Greene.) 



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

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

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

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

 'terminating ca3cally before quite reaching the oral extremity.' 

 The second pair of canals (i) 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 (&), and these, in turn, give 

 rise to eight 'tertiary' radial canals (Z), 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 cascallyboth 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 



