GENERAL SKETCH OF ACALEPHS. 23 



Acalephs in early geological periods, the gelatinous texture 



of the ordinary Jelly-fishes making their preserva- 



tion almost impossible. It is not strange that the 



true nature of this animal should have remained 



so long unexplained ; for it is only by the soft 



parts of the body, not of course preserved in the 



fossil condition, that their relations to the Acalephs 



* A 



may be detected ; and they are so shy of approach, 



drawing their tentacles and the upper part of the 



body into their limestone frame if disturbed, that it is not 



easy to examine the living animal. 



The Millepore is very abundant on the Florida reefs. From the 

 solid base of the coral stock arise broad ridges, branching more or 

 less along the edges, the whole surface being covered by innu- 

 merable pores, from which the diminutive animals project when 

 expanded. (Fig. 25.) The whole mass of the coral is porous, 

 and the cavities occupied by the Hydras are sunk perpendicularly 

 to the surface within the stock. Seen in a transverse cut these 

 tubular cavities are divided at intervals by horizontal partitions 

 (Fig. 26), extending straight across the cavity from wall to wall, 

 and closing it up entirely, the animal occupying only the outer- 

 most open space, and building a new partition behind it as it 

 rises in the process of growth. This structure is totally different 

 from that of the Madrepores, Astrseans, Porites, and indeed, from 

 all the polyp corals which, like all Polyps, have the vertical par- 

 titions running through the whole length of the body, and more 

 or less open from top to bottom. 



The life of the Jelly-fishes, with the exception of the Mille- 

 pores and the like, is short in comparison to that of other Radi- 

 ates. While Polyps live for many years, and Star-fishes and 

 Sea-urchins require ten or fifteen years to attain their full size, 

 the short existence of the Acaleph, with all its changes, is accom- 

 plished in one year. The breeding season being in the autumn, 

 the egg grows into a Hydroid during the winter ; in the spring the 

 Jelly-fish is freed from the Hydroid stock, or developed upon it as 

 the case may be ; it attains its full size in the fall, lays its eggs 



Fig. 26. Transverse section of a branch, showing pits, a a a a, of the large Ilydroids with the hori- 

 zontal floors. (JJyassiz.) 



