200 MANUAL OF ZOOLOGY. 



As regards their development, most of the Echinoids pass 

 through a metamorphosis, as spoken of previously in treating 

 of the development of the class. In these cases the larva is so 

 unlike the adult animal that it was originally described as a 

 distinct animal under the name of Pluteus, from- its resem- 

 blance to a painter's easel (fig. 92). The larva exhibits bila- 

 teral symmetry, and is furnished with provisional organs in 

 the shape of ciliated epaulettes, a skeleton of calcareous rods, 

 and an alimentary canal. The adult Echinoid is developed 

 out of a portion of its substance only; and the rest of the 

 larva is absorbed or thrown off. In some Echinoids, on the 

 other hand, as we have seen, the process of development is 

 direct, and there is no " Pluteus " stage, but the young animal 

 is produced viviparously, and simply requires to grow to be 

 converted into the adult. 



The typical Sea-urchins are divided into the two great groups 

 of the " Irregular " and " Regular " Echinoids (or the -Echinoi- 

 dea exocyclica and Echinoidea endocyclica). The Irregular Echin- 

 oids have the anus situated outside the apical disc, marginal 

 or submarginal in position, and have only four genital plates. 

 They are also mostly destitute of a masticatory apparatus ; and 

 are generally of an oblong, pentagonal, heart-shaped, or dis- 

 coidal figure (as in the common " Heart-urchins " and " Cake- 

 urchins"). The "Regular" Echinoids, on the other hand, 

 have the anus placed at the summit of the test, surrounded by 

 the genital disc; the test is almost always circular or sphe- 

 roidal ; and the mouth is armed with a complicated mastica- 

 tory apparatus. 



Another singular group is that of the EchinothuridcB, in which 

 the test is "regular," but the plates of both the ambulacral and 

 inter-ambulacral areas are imbricated and overlap one another, 

 rendering the test quite flexible. The existing genera, Asthe- 

 nosoma (or Calveria) and Phormosoma, and the Cretaceous 

 genus Echinothuria belong to this group. 



A fourth group of the Echinoids is that of the Perischoechi- 

 nida, which is not only extinct, but is wholly confined to the 

 Palaeozoic period. In all these ancient forms there is the pecu- 

 liarity that the test consists of more than twenty rows of plates, 

 there being a multiplication of either the inter-ambulacral or 

 the ambulacral plates, though there are still only five inter- 

 ambulacral and five ambulacral areas. Thus in Archceodaaris, 

 Paltzchinus, Lepidechinus, and Eocidaris, the ambulacral areas 

 agree with those of the recent Urchins in being composed of 

 only two rows of plates ; whilst there are from three to eight 

 or more rows of plates in each inter-ambulacral area. On 



