LARVAL ECHINI 



899 



feet ' of the adult. The disc gradually extends itself over the stomach, 

 and between its tubules the rudiments of spines are seen to protrude 

 ( I )) ; these, with the tubules, increase in length, so as to project against 

 the envelope of the pluteus, and to push themselves through it ; whilst, 

 at the same time, the original angular appendages of the pluteus 

 diminish in size, the 

 ciliary movement be- 

 comes less active, being 

 .superseded by the action 

 of the suckers and spines, 

 and the mouth of the 

 pluteus closes up. By 

 the time that the disc- 

 has grown over half of 

 the gastric sphere, very 

 little of the pluteus re- 

 mains, except some of 

 the slender calcareous 

 rods, and the number 

 of suckers and spines 

 rapidly increases. The 

 calcareous framework of 

 the shell at first consists, 

 like that of the star- 

 fishes, of a series of 

 isolated networks de- 

 veloped between the 

 cirrhi, and upon these 

 rest the first formed 

 spines (D). But they 

 gradually become more 

 consolidated, and extend 

 themselves over the 

 granular mass, so as to 

 form the series of plates 

 constituting the shell. 

 The mouth of the Echi- 

 nus (which is altogether 

 distinct from that of the 

 pluteus) is formed at 

 that side of the granular 

 mass over which the 

 shell is last extended ; 

 and the first indication 

 of it consists in the ap- 



B 



584. Embryonic development of Ecliintts: A 

 Pluteus larva at the time of the first appearance 

 of the disc : a, mouth, in the midst of the four- 

 pronged proboscis ; b, stomach; c, Echinoid disc: 

 (1, d, (1, cl, four arms of the platens-body; r, cal- 

 careous framework ; /', ciliated lobes : //, </, c/, ff, 

 ciliated processes of the proboscis ; B, disc with 

 the first indication of the sucking-feet; C, disc, 

 with the origin of the spines between the tubular 

 sucking-feet ; D, more advanced disc, with the feet, 

 <7, and spines, x, projecting considerably from the 

 surface. (N.B. In B, C, and D, the Pluteus is not 

 represented, its parts having undergone no change, 

 save in becoming relatively smaller.) 



pearance of the five cal- 

 careous concretions, which are the summits of the five portions of 

 the framework of jaws and teeth that surround it. All traces of 

 the original pluteus are now lost; and the larva, which now 

 presents the general aspect of an Echinoid animal, gradually 

 in size, multiplies the number of its plates, cirrhi. and 



3 M 2 



augments 



