156 MANUAL OF ZOOLOGY. 



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 Ophiuridea 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. 



57, i.) 



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 

 anal aperture ; the whole being " disposed in a longitudinal 

 and vertical plane, dividing the larval body into two symme- 

 trical halves." Besides the digestive canal, no other organs 

 have hitherto been discovered in these larvae. In the further 

 process of development, " an involution of the integument 

 takes place upon one side of the dorsal region of the body, 

 so as to give rise to a caecal tube, which gradually elongates 

 inwards, and eventually reaches a mass of formative matter, 

 or blastema, aggregated upon one side of the stomach. 

 Within this, the end of the tube becomes converted into a 

 circular vessel, from which trunks pass off, radially, through 

 the enlarging blastema. The latter, gradually expanding, 

 gives rise, in the Echinidea, the Asteridea, the Ophiuridea, and 

 the Crinoidea, to the body-wall of the adult ; the larval body 

 and skeleton (when the latter exists), with more or less of the 

 primitive intestine, being either cast off as a whole, or dis- 

 appearing, or becoming incorporated with the secondary 

 development, while a new mouth is developed in the centre of 

 the ring formed by the circular vessel. The vessels which 

 radiate from the latter give off diverticula to communicate 

 with the cavities of numerous processes of the body the so- 

 called feet which are the chief locomotive organs of the 

 adult. The radiating and circular vessels, with all their 

 appendages, constitute what is known as the ' ambulacral 

 system ; ' and in Asterids and Echinids this remarkable system 

 of vessels remains in communication with the exterior of the 

 body by canals, connected with perforated portions of the 

 external skeleton the so-called ' madreporic canals,' and 

 'tubercles.' In Ophitirids the persistence of any such com- 



