366 ECHINODERMATA [CH. f 



which also develops into a hydrocoele. The coelom on the right i 

 side of the larva of Holothuroidea is not segmented at all. The i 

 madreporic vesicle arises as an outgrowth from the most anterior i 

 section of the coelom on the right side, very near the mid-dorsal i 

 line. The most posterior divisions form the body-cavity of the i 

 adult; the left one grows in a ring-shaped manner, encircling, as 

 with a wider ring, the ring of the hydrocoele, while through the 

 centre of both rings the oesophagus of the adult grows. The 

 oesophagus of the larva is usually cast off, but sometimes, as in 

 the Ophiuroidea, it is directly converted into that of the adult by 

 being shifted to the left before the rosette of the water-vascular 

 system becomes a ring. 



The Dipleurulae of the Asteroidea fix themselves at the conclusion 

 of their larval life by the anterior end of the body, using the prae- 

 oral lobe as a stalk. The fixed stage is omitted in Ophiuroidea, 

 Echinoidea and Holothuroidea, but the larva of Antedon rosacea 

 the only Crinoid whose development is known also converts the 

 prae-oral lobe into a stalk. But in the case of the Asteroidea the body 

 becomes bent on the stalk in such a way that the stalk springs from 

 close to the mouth of the adult. The stalk is eventually absorbed 

 and the star-fish commences its adult life, breaks loose from its 

 attachment and moves away. In the Crinoid, on the other hand, the 

 mouth becomes rotated away from the stalk, and the latter seems to 

 spring from the aboral surface. 



The whole of this development seems to point to the conclusion 

 that the radially symmetrical Echinodermata are derived from a 

 bilaterally symmetrical ancestor- with traces of metameric segmen- 

 tation ; that the acquisition of radial symmetry was due in the first 

 instance to the assumption of a fixed mode of life, followed by the 

 dwindling of the organs of the right side and the compensating 

 greater growth in those of the left. The Crinoidea seem to have 

 retained the original mode of feeding by means of the current 

 produced by cilia, and in them the mouth became shifted upwards 

 away from the stalk into a position favourable for the capture 

 of floating prey. In the Asteroidea and the other Eleutherozoa 

 food is obtained by seizing it with the tube-feet, and hence in 

 these the mouth was bent downwards so that the stalk seems to 

 spring from the oral surface. 



This development is not only interesting on account of the 

 extraordinary metamorphosis which the young undergo, but also on 

 account of the fact that whilst the adult is utterly unlike any of the 

 other Coelomata, the structure of the young is reconcilable with the 





