IN THE ANIMAL KINGDOM. 85 



attachment, as it has a mouth and gastric cavity of its own. 

 The back of the ciliated body assumes a polygonal form, and 

 is directly transformed into the starfish, which developes a 

 new mouth in the centre of the ventral surface, a little to 

 one side of the pedicle, the latter appendage gradually dis- 

 appearing by a process of absorption. 



But in the great majority of the class there is in the 

 course of development a marked change, of the nature of 

 alternation. In all these cases, a decided breach occurs in 

 the continuity of the process, by the establishment of a new 

 focus of organization ; and in not a few there is a breach 

 of structure also, in the mechanical separation of the later 

 from the pre-existing growth. The first steps of develop- 

 ment — those which directly affect the germ — consist in the 

 formation of an alimentary canal, with oral and anal open- 

 ings, and in the disappearance of the cilia on the extemal 

 surface, except in particular spots, especially along a circle 

 surrounding the oral region. These are follow^ed by certain 

 alterations of external form and configuration, differing in 

 the different sections of the class, and distinguished by par- 

 ticular names. That characteristic of the Holothuridce has 

 been termed Auricular ia, and is of a cylindrical or barrel 

 shape, girt with numerous ciliated rings, and not unlike the 

 larva of some Annelida. That of the Asteridse, known 

 under the name of Bipinnaria, attains a considerable size — 

 an inch or more in length — and assumes a very extraordi- 

 nary form, from the development of the ciliated region in 

 front of the mouth, wdiich throws out several long processes 

 on each side, and at the anterior extremity two fin-like ex- 

 pansions, placed one above the other. But the most ela- 

 borate specimen of such provisional organization is presented 

 to us in the Pluteus — the form from which the Echinida? 

 and Ophiuridae are derived. The general form of the 

 Pluteus is that of a quadrilateral pyramid, dome-shaped 

 above and slightly excavated at the base, the corners oi 



