HISTORY OF C@LENTERA 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 Beroé and its near relatives, there 
are two retractile tentacles. All are hermaphrodite. The development 
Fic. 92.—Aydroctena. A medusoid with suggestions: 
of Ctenophore structure, but a true medusoid none the less. 
aé.p., Aboral sensory organ; 7., retractile tentacle; 
v., velum ; JZ., mouth; S7Z7., stomach. 
1 
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 Cztenoplana and Celoplana, two 
curious flattened forms which crawl like Planarians. Mortensen’s 
remarkable sessile 77a/fed/a corroborates this affinity. 
Examples :— ° : 
(a) With tentacles, Cydpfe and the ribbon-shaped Venus’ Girdle 
(Cestum venerts). (6) Without tentacles, Beroé. 
History of Ceelentera.—Of corals, as we would expect, the rocks 
preserve a faithful record, and we know, for instance, that in the 
older (Palzeozoic) 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 
