SEA-URCHINS. 



163 



absorbed, and the spines and suckers of the young sea-urchin inci'ease in size and num- 

 ber. When the pluteus has finally disappeared, the young sea-urchin is more like the 

 adult than is the young star-fish, but the plates of the apical region are not only more 

 conspicuous in relation to the size of the test, but differ somewhat in their arrange- 

 ment from those of the adult. The anus is at first wanting, and the anal plate is rela- 

 tively large, is in the centre of the apical area, and is united by its edges with the five 

 plates which, though imperforate in the young, become the genital plates in the adult. 

 The other five plates that surround the apical system (the ocular plates) are also im- 

 perforate, and from a circle outside that formed by the genital plates, the spaces 

 between them being occupied by interambulacral plates. In this stage the homology 



Fig. 142. — Old pluteus of Arbacia, with developing sea-urchin. 



between the apical region of a sea-urchin, and the calyx of a crinoid is strongly brought 

 out ; the anal plate representing the basalia, the genital plates the parabasalia, ^nd the 

 ocular plates the first radials. The ambulacra may therefore be taken to represent the 

 arms of a crinoid, and the interambulacral plates of the echinids are homologous with 

 the interradial plates of the Crinoidea. The calcareous skeleton of the pluteus under- 

 goes resorption, but the remainder of the larva passes into the growing sear-urchin. 



The pluteus form is not universal among echinoids, since several forms from 

 the southern hemisphere (Ilemiaster philippii, IT. cavernosus, Anochanus sinensis, 

 Cidaris nutrix, etc.), develop directly into sea-urchins without, or with only traces of, 

 a metamorphosis. Notwithstanding this great difference in the mode of development, 

 the species which develop directly are very nearly related to species which live in the 

 northern seas and pass through the pluteus stage. The ' Challenger ' expedition did 



