466 DEVELOPMENT OF STAR-FISH 



wards them and take them up again, an exertion of volition 

 which we should hardly have expected to meet with in creatures 

 so low in the scale of organization as these. In one species that 

 has been observed, the cavity for the reception of the eggs is 

 situated under the skin of the back. The egg gives birth to a 

 minute ciliated creature, which swims about in the water con- 

 tained in the cavity of its parent, but in most cases it soon ac- 

 quires from one to four little processes or prehensile arms, by 

 which it is enabled to cling to the walls of this cavity. When 

 detached, however, it still swims about freely by the agency of 

 cilia. In the simplest mode of development this larva then 

 gradually acquires a radiate form, although a greater or less por- 

 tion of the larval organs are changed in the course of de- 

 velopment ; the stomach being usually, as in the Echinida, the 

 most important organ of the larva that is transferred to the per- 

 fect Star- fish. In Ophiura, however, we meet with a mode of 

 development as remarkable as that of the Echinida ; the full- 

 grown larvse of this genus present nearly the same easel-like 

 form as those of the Sea-Urchins, and, like them, were formerly 

 described as a peculiar animal form under the generic name of 

 Pluteus. From this singular larva the perfect Ophiura is pro- 

 duced in the following manner. Around the stomach and oeso- 

 phagus of the Pluteus small sacs make their appearance, at first 

 contained within its body, but gradually increasing in size until 

 they form the disc of the future Star-fish, suspended, as it were, 

 from the lower part of the bell of the larva. Indications of the 

 arms then present themselves ; calcareous matter begins to be de- 

 posited in the skin ; the larval mouth is displaced, and a new 

 one is formed ; the young Star-fish continues increasing in size, 

 until at last the larva is got rid of altogether, the only part of it 

 which remains in the perfect Star-fish being the stomach, 



1105. The mouth of the Asterice opens at once into the 

 stomach ; it is unprovided with teeth ; and the digestive secre- 

 tions appear to constitute the only means possessed by the 

 animal of reducing its food. There is considerable variety in 

 this group, as to the number of rays, the proportion which they 

 bear to the central disc, and the degree in which the stomach 



