124 MARINE ANIMALS OF MASSACHUSETTS BAY. 



structure of these animals so individual a character, that some 

 naturalists, overlooking the law of radiation, as prevalent in them 

 as in any members of this division, have been inclined to separate 

 them, as a primary division of the animal kingdom, from the 

 Polyps and Acalephs, in both of which the body-wall furnishes 

 the walls of the different internal cavities, either by folding in- 

 wardly in such a manner as to enclose them, as in the Polyps, 

 or by the cavities themselves being hollowed out of the general 

 mass, as in the Acalephs. 



Star-fish. (Astracanthim.) 



The egg of the Star-fish, when first formed, is a transparent, 

 spherical body, enclosing the germinative vesicle and dot. (See 

 Pig. 155.) As soon as these disappear, the segmentation of the 

 yolk begins ; it divides first into two portions (see Fig. 156), 

 then into four, then into eight, and so on ; but when there are 

 no more than eight bodies of segmentation (see Fig. 157), they 



Fig. 155. Fig. 156. Fig. 157. 



already show a disposition to arrange themselves in a hollow 

 sphere, enclosing a space witliin, and by the time the segmenta- 

 tion is completed, they form a continuous spherical shell. At this 

 time the egg, or, as we will henceforth call it, the embryo, escapes 

 and swims freely about. (See Fig. 158.) The wall next begins 

 to thin out on one side, while on the opposite side, which by com- 

 parison becomes somewhat bulging, a depression is formed (wa, 

 Fig. 159), gradually elongating into a loop hanging down within 

 the little animal, and forming a digestive cavity, (c?, Fig. 160.) 

 At this stage it much resembles a young Actinia. The loop 

 spreads somewhat at its upper extremity, and at its lower end is 



Fig. 155. Egg of Star-fish. 



Fig. 156. Egg of Star-fish in which the yolk has been divided into two segments. 



Fig. 157. Egg in which there are eight segments of the yolk. 



