208 ZOOLOGY. 



CLASS III. ECHINOIDEA. 



Spherical, Jwart-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 five teeth ; am- 

 bulacralfeet 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. Melonilida (Melonites, Protechinus, 

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

 daris). 



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

 Suborder 1. Desmosticha (Cidaris, Echinus, Strongylocen- 

 trotus, Echinometra, Clypeaster, and Echinarachnius). 

 Suborder 2. Petalosticha (Echinobrissus, Anochanus, 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 ambulacral, 

 interambulacral, and genital plates. 



CLASS IV. HOLOTHUROIDEA (Sea-cucumbers}. 



General Characters of Holoth.urians. We now come to 

 Echinoderms 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 Eupyrgus, 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 Sipunculus ; 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 affiniti 

 (though strong analogies) with the Coelenterates, or the re- 





