2o6 



PHYLUM CCELENTERA 



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

 The mesodermic layer is 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 

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

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

 Polyclad worms, especially through Ctenoplana and Coeloplana, two 



Fig. III. — Diagram of a Ctenophore. — After Chun. 



M., Mouth ; 5., sensory organ ; T., tentacle cut short ; SH., 

 pouch of tentacle ; C, ciliated combs ; F., funnel or central canal ; 

 SV., paragastric canal running parallel with stomodaeum ; G., 

 other canals of the gut ; V., one of the meridional canals, bear- 

 ing gonads, running under the bands of ciliated combs. 



curious flattened forms which crawl like Planarians. Mortensen's 

 remarkable sessile Tjalfiella corroborates this affinit3\ 



Examples : — 



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

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



History of Coelentera. — 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 



