100 



INVERTEBRATE ANIMALS. 



alluded to here. In its later stages it was originally described 

 as a distinct animal under the name of " Pluteus^ In this 

 state the larva is a curious, easel-shaped body, with a distinct 

 alimentary canal and an internal calcareous skeleton, and ex- 

 hibiting distinct bilateral symmetry. The remarkable point, 

 however, about its further development is, that the young 

 Echinus is developed out of only a portion of the Pluteus, and 

 the greater part of the latter, including the skeleton, is cast 

 away as useless. 



The majority of the sea-urchins are found at moderate 

 depths in the sea, especially in the neighborhood of oyster- 

 banks. Others spend their existence buried in the sand ; and 

 one species excavates holes for itself in the solid rock, ap- 

 parently by some mechanical action. 



OKDER II. ASTEROIDEA (Gr. aster, star; eidos, form). 

 As the structure of the sea-urchins may be taken as embody- 

 ing the most important anatomical peculiarities of the Echino- 

 dermata, and as this has been described at some length, it will 

 not be necessary to do more than briefly indicate the more 

 important characteristics of the remaining orders. In the 

 present order are included all the true star-fishes, the sand- 

 stars and brittle-stars being generally regarded as a distinct 





FIG. SS.Cribetta oculata (after Forbes). 



group. The body in all the Asteroidea is more or less ob- 

 viously star-shaped (Fig. 33), consisting of a central disk sur- 

 rounded by five or more lobes or arms, which radiate from the 



