122 MANUAL OP ZOOLOGY. 



CLASS I. ECHINODERMATA. 



The members of this class are known commonly as Sea- 

 urchins, Star-fishes, Brittle- stars, Feather- stars, Sea-cucumbers, 

 &c., and the following are their leading characteristics. They 

 are all animals which, in the adult condition, show a more or 

 less distinctly radiate condition of their parts, especially of 

 those around the mouth ; whilst in their embryonic stages 

 they are more or less distinctly bilaterally symmetrical. The 

 external envelope of the body (' perisome') is either composed 

 of numerous calcareous plates, articulated together, or of a 

 coriaceous integument, in which calcareous granules and 

 spicules are usually developed. In all adult Echinoderms there 

 is a system of tubes, termed the * ambulacral system,' which 

 subserves locomotion, and usually communicates with the 

 exterior. An alimentary canal is always present, and is com- 

 pletely shut off from the body-cavity. In many, if not in all, 

 both neural and ha3mal systems are developed. The nervous 

 system in all the adult EcMnoderms is a ring-like gangliated 

 cord, which surrounds the oesophagus, and sends branches 

 parallel to the radiating ambulacral canals. 



The special features of the structure of fheEchinodermata will 

 be noticed under each order, but it will be as well to give here 

 an abstract of Professor Huxley's description of the process of 

 development in the members of the class. In the great 

 majority, if not in all, of the Echinodermata the impregnated 

 ovum is developed into a free- swimming, ciliated, ovoid embryo. 

 Soon the cilia become restricted to one, two, or more bands, 

 which are generally disposed transversely to the long axis of 

 the body, and are in all cases bilaterally symmetrical. The 

 parts of the body which support the cilia are usually developed 

 into protuberances, or processes, which are symmetrically dis- 

 posed upon the two sides of the body. ' The larvae of Asteridea 

 and Holothuridea are devoid of any continuous skeleton, but 

 those of Opliiuridea and Echinidea possess a very remarkable, 

 bilaterally symmetrical, continuous, calcareous skeleton, which 

 extends into, and supports the processes of the body.' In this 

 stage the larva form of the two orders last mentioned was 

 described by Miiller as a distinct animal, under the name of 

 Pluteus, from its resemblance to a painter's easel. (See 

 fig. 33, 1.) 



An alimentary canal soon appears in the larva, forming a 

 curve with an open angle towards the ventral surface of the 

 organism. The parts of the alimentary canal consist of a mouth, 

 gullet, globular stomach, and short intestine, with a distinct 



