41 6 CTENOPHORA 



passes a single pair of blind canals, the " paragastric canals," 

 one on each side of the stomodaeum, to end in the oral cone. 



In the Lobata the paragastric canals communicate with the 

 longitudinal canals under the transverse costae,^ and send long 

 blind processes into the lobes. In the Cestoidea the arrange- 

 ment of the canals is considerably modified in adaptation to the 

 needs of the ribbon-like body. In the Beroidae the paragastric 

 and longitudinal canals are in communication by a peripheral 

 network of canals, and in the Platyctenea there is also a net- 

 work of canals but without any definite longitudinal vessels. 



Sexual Organs. — -Most of the Ctenophora are undoubtedly 

 hermaphrodite, but Willey was unable to find ova in some of his 

 specimens of Ctenoplana that were producing spermatozoa. In 

 the Cydippidea the ova are produced on one side of the longitu- 

 dinal canal and the spermatozoa on the other. Each longitudinal 

 canal therefore performs the functions of a hermaphrodite gland. 

 When the sexual cells are ripe they escape into the infundi- 

 bulum and are discharged by the mouth. In Gtenoplana there 

 are definite and direct male genital ducts. 



The ova are very small when discharged and undergo com- 

 plete segmentation in the sea water. The development of the 

 Cydippidea is really direct, but there is a stage passed through 

 in which the tentacles are relatively very prominent and 

 situated close to the aboral pole, and this stage is very different 

 in appearance from the adult. In the Lobata and Cestoidea 

 there is, however, a definite larval stage, of the general appear- 

 ance of a Mertensia, and during this stage fertile eggs and 

 spermatozoa are formed and set free. 



Distribution. — Ctenophora are found at the surface of 

 nearly all seas, and many of the genera have a cosmopolitan 

 distribution. Some of the Lobata, the Cestoidea, and the 

 Platyctenea are more commonly found in the warmer regions of 

 the world. Pleurobrachia pileus, Bolina infundibulum, Beroe 

 ovata, and B. cucumis occur off the British coast. 



Most of the Ctenophora are from 5 to 20 mm. in diameter, 

 but Beroe reaches the length of 90 mm., Eucharis multicornis 



' The two costae that are seen in the middle when the Ctenophore is viewed in 

 the transverse plane, as in Figs. 180 and 181, and the corresponding costae on the 

 opposite side are called the "transverse" costae; the other four are called the 

 " sagittal" costae. 



