490 OF ECHINODEEMATA. 



nus from its "pluteus," is given by the formation of a circular 

 disk (Fig. 244, a, e), on one side of the central stomach (b) ; and 

 this disk soon presents five prominent tubercles (b), which sub- 

 sequently become elongated into tubular cirrhi. The disk gradu- 

 ally extends itself over the stomach, and between its cirrhi the 

 rudiments of spines are seen to protrude (c); these, with the 

 cirrhi, 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 becomes less active, being 

 superseded by the action of the cirrhi and spines, and the mouth 

 of the " pluteus" closes up. By the time that the disk has grown 

 over half of the gastric sphere, very little of the "pluteus" re- 

 mains, except some of the slender calcareous rods ; and the num- 

 ber of tentacula 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 developed 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. The mouth 

 of the Echinus (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 appearance of five calcareous 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 augments in size, multi- 

 plies the number of its plates, cirrhi, and spines, evolves itself 

 into its particular generic and specific type, and undergoes various 

 changes of internal structure, tending to the development of the 

 complete organism. In collecting the free-swimming larvae of 

 Echinodermata, a fine muslin net should be employed in the 

 manner already described (§ 306); and the search for them is of 

 course most likely to be successful in those localities in which 

 the adult animals of the respective species abound, and on warm 

 calm days, in Avhich they seem to come to the surface in the 

 greatest numbers. 



324. It is remarkable that the Comatida, one of the most active 

 of all Echinoderms in its adult state, passes a portion of the early 

 period of its life in a fixed condition ; being attached by a stem 

 to sea-weeds or zoophytes, preciselj^ after the manner of the Ori- 

 noids or "lily-stars," which were the most common types of this 

 class in the older epochs of the world's history. In this phase of 

 its life, which was first discovered by Mr. J. V. Thompson, of 

 Cork, in 1823, it is very minute, and forms a most beautiful ob- 

 ject for the lower magnifying powers, when viewed in fluid by a 

 strong incident hght, as nearly as possible in its natural condition. 

 It has hitherto been found so rarely, however, that few Micro- 



