IV 



PHYLUM CCELENTERATA 



221 





oral edge. The young of Cestu& (B) resembles a compressed Cydippid which 

 undergoes gradual elongation in the median plane. 



Beroe, the principal genus of the Beroida, has the form of a cylinder (Fig. 

 172), one end of which is rounded and bears the sense-organ, the other truncated 

 and occupied entirely by the immense mouth 

 (mth.). The greater part of the body is taken 

 up by the huge stomodseum ; the infundibulum 

 (inf.), per-radial and infundibular canals, &c., all 

 being crowded into a small space at the aboral 

 pole. The meridional canals send off branches 

 which unite with one another, forming a com- 

 plex network of tubes, and at their oral ends 

 the four meridional canals of each (right and left) 

 side and the corresponding stomodseal canal 

 unite into a horizontal tube, which runs parallel 

 with the margin of the mouth. There is no 

 trace of tentacles either in the adult or in the 

 embryonic condition. 



The Platyctenea are represented by four 

 genera Ctenoplana, Ccdoplana, Tjalfidla and 

 Gastrodes. 



Ctenoplana (Fig. 173) is a small marine 

 animal, nearly circular in outline, flattened 

 dorso- vent rally, and about 6 mm. in diameter. 

 It has hitherto been found only twice once off 

 the west coast of Sumatra and once among the 

 islands to the east of Papua. While able to swim 

 freely, it is able also to creep on its ciliated oral 

 or ventral surface. When it is swimming the 

 animal draws downwards the edges of the disc 

 so that it becomes somewhat helmet-shaped 

 when viewed laterally. The organs of locomo- 

 tion are eight small, deeply sunk swimming 

 plates. In the centre of the aboral or dorsal 

 surface is a polar body or sense-organ with a 

 statolith, surrounded by a ring of small ciliated 

 tentacles which are disposed bilaterally with 

 reference to the transverse plane, but without 

 polar-plates (ciliated areas). Nerve-centres in the 

 form of several ganglia are situated in close relation to the polar body. The 

 mouth is in the centre of the ventral (oral) surface. There are two pinnate 



ctr. 



mih 



FIG. 172. Beroe forskalii. 

 inf. infundibulum ; mth. 

 mouth ; s.pl. swimming- 

 plates. (After Chun.) 



FIG. 173. Ctenoplana kowalevskii, dorsal aspect, t, t. tentacles ; tsh. tentacle sheaths ; ctr. 

 subtransverse costse; ess. sub-sagittal costse ; st. "stomach" (? stomodseum), 1, 2, 3, 4, 

 the four principal lobes of the infundibulum ; pf. sensory tentacles representing the 

 polar plates ; pg. pigment spots. (After Wllley.) 



