HISTORY OF CCELENTERA 



175 



like combinations of cilia. The stinging cells are almost always 

 replaced by "adhesive cells." The mouth is at one pole, and leads 

 into an ectodermic gullet. The gastric cavity is usually much branched. 

 The mesenchyme is very well developed, and includes muscular and 

 connective cells. At the aboral pole there is a sensory organ, including 

 an "otolith," which seems of use in steering. Here, also, there are 

 two excretory apertures. Except in Beroe and its near relatives, there 

 are two retractile tentacles. All are hermaphrodite. The development 



FIG. 92. Hydroctena. A medusoid with suggestions 

 of Ctenophore structure, but a true medusoid none the less. 



ab.o.> Aboral sensory organ ; T., retractile tentacle ; 

 z>., velum; M., mouth; ST., stomach. 



is direct. They are pelagic, very active in habit, carnivorous in diel, 

 and often phosphorescent. According to some, they lead on to 

 Polyclad worms, especially through Ctenoplana and C&loplana, two 

 curious flattened forms which crawl like Planarians. Mortensen's 

 remarkable sessile Tjalfiella corroborates this affinity. 



Examples : 



(a) With tentacles, Cydippe and the ribbon-shaped Venus' Girdle 

 (Cesium veneris). (b) Without tentacles, Beroe. 



History of Coalentera. Of corals, as we would expect, the rocks 

 preserve a faithful record, and we know, for instance, that in the 

 older (Palaeozoic) strata they were represented by many types. We 

 often talk of the imperfection of the geological record, and rightly, for 

 much of the library has been burned, many of the volumes are torn, 

 whole chapters are wanting, and many pages are blurred. But this 

 imperfect record sometimes surprises us, as in the quite distinct remains 

 of ancient jelly-fish, which animals, as we know them now, are appar- 

 ently little more than animated sea-water. We should also grasp the 

 conception, with which Lyell first impressed the world, of the uniformity 

 of natural processes throughout the long history of the earth. Thus in 

 connection with Coelentera we learn that there were great coral reefs in 



