126 ZOOLOGY. 



CLASS III. ECHINOIDEA. 



Spherical, heart-shaped, or disk-like Echinoderms, with a solid shell of im- 

 movable plates, bearing interambulacral spines; with a mouth and anal 

 opening, the mouth in most of the species armed with Jive teeth; am- 

 bulacral feet well developed. The sexes distinct. Development either direct, 

 or, as in most cases, by a marked metamorphosis from a pluteus larva. 



Order 1. Palechinida. Shell composed of more than twenty rows of 

 plates. Suborder 1. Melonitida (Melonites, Protechinus, 

 Palsechinus, Archaeocidaris). Suborder 2. Eocidaria (Eoci- 

 daris). 



Order 2. Autechinida. Shell composed of twenty rows of pfetes. 

 Suborder 1. DesmosticJ.a (Cidaris, Echinus, Strongylocen- 

 trotus, Echinometra, Clypeaster, and Echinarachnius). 

 Suborder 2. PetalosticJia (Echinobrissus, Anochauus, Pour- 

 talesia, Spatangus, and Schizaster). 



Laboratory Work. "We have already given some hints as to the 

 mode of dissecting sea-urchins, which should be done under water in 

 deep pans. Great care must be taken in removing the digestive canal, 

 -which is very delicate in itself, and usually filled with sand. In study- 

 ing the water- vascular and blood-vessels, careful, skilful injections with 

 .carmine are indispensable. The spines may be studied by making thin 

 longitudinal and transverse sections. The test, or shell, should be de- 

 nuded of the spines in order to study the relations of the arnbulacral, 

 interambulacral, and genital plates. 



CLASS IV. HoLOinuROiDEA (Sea-cucumbers}. 



General Characters of Holothurians. We now come to 

 l^chinoderms in which the body is usually long, cylin- 

 drical, with a tendency to become worm-like, and in cer- 

 tain genera, as Synapta, Chirodota, and Eiqjyrgus, it is 

 difficult both in their larval stages (Synapta) and in the 

 external and internal anatomy of the adults to separate 

 them from worms like Sipuncuhis ; authors have therefore 

 -been led to the adoption of one of two views : first, either 

 that the worms and Echinoderms have had a common origin, 

 and the latter, though truly radiate, have no near affinities 

 ^though strong analogies) with the Ccelenterates, or the re- 



