THE CTENOPHORA 17 



and a ventral or oral surface. It has eight very short ribs with 

 the characteristic combs, which can be withdrawn into or evagin- 

 ated from pouch-like cavities in the body wall. There is a single 

 pair of pinnate tentacles retractile within tentacle sheaths ; the 

 tentacles are solid and muscular. In the centre of the aboral 

 surface of the body is a sense organ, consisting of an otolith mass 

 suspended by stiff cilia, and two crescentic rows of ciliated ten- 

 tacles or papillae, which are evidently homologous with the polar 

 fields, and recall the lappet-like processes of the edge of the polar 

 fields of the Beroidae (Fig. X. pf). The mouth is circular and 

 leads into a " stomach," which is compressed in the sagittal plane ; 

 it is not known whether the " stomach " is a stomodaeum. An 

 infundibular vessel passes from the aboral end of the stomach 

 towards the sense organ, which it embraces without opening to 

 the exterior. From each of the two flattened sides of the 

 stomach a narrow canal, lying in the transverse plane, leads into 

 a pair of saccular lobes, and from these numerous diverticula are 

 given off forming a peripheral canal system. These peripheral 

 canals may be compared with the canals of the lobes of Lobatae. 

 The testes are situated at the bases of the two saccular lobes at 

 either end of the main transverse canal of the gastrovascular 

 system, and they have ducts which open to the exterior just below 

 the costae. The ovaries have not been observed. Ctenoplana 

 either swims by means of its combs, or crawls on the bottom by 

 its ventral surface. It can also attach itself, like a Planarian, 

 ventral surface uppermost, to the surface film of the water. Its 

 body is thickened in the transverse plane, and the sagittal margins 

 are produced into two thin rounded lobes. In swimming the 

 lobes are folded together like the leaves of a book. It should be 

 noticed that the lobes of Ctenoplana correspond in position with 

 those of the Lobatae. The ventral surface of Ctenoplana is ciliated, 

 but, excepting for the costae and sensory tentacles, there are no 

 cilia on the dorsal surface. 



Unfortunately we have only a meagre account of the anatomy 

 of Coeloplana. It appears, in general, to resemble Ctenoplana, but 

 has no costae, and the whole surface of the body is uniformly 

 ciliated. Both Ctenoplana and Coeloplana have been said to exhibit 

 remarkable Planarian affinities because of their dorso-ventrally 

 flattened bodies, their crawling habits, and the ciliation of the 

 ectoderm, partial in the case of Ctenoplana, complete in the case of 

 Ceoloplana. Not much weight can be attached to these characters. 

 Habit is a very insecure guide to affinity. One of the Cydippidae, 

 Lampetia pancerina, crawls on its oral surface, everting the stomo- 

 daeum so as to form a broad creeping surface. The flattened 

 bodies of Ctenoplana and Coeloplana are clearly correlated with the 

 adoption of the creeping habit already foreshadowed in Lampetia. 



