208 ZOOLOGY. 
Cuass ITI.—ECHINOIDEA. 
Spherical, heart-shaped, or disk-like Echinoderms, with a solid shell of tm- 
movable plates, bearing interambulacral spines; with a mouth and anal 
opening, the mouth in most of the species armed with five 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, 
Palechinus, Archzocidaris). Suborder 2. Hocidaria (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. 
Crass IV.—Ho.LorHurRoIpDEa (Sea-cucumbers), 
General Characters of Holothurians._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 Hupyrgus, 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 Sipwnculus ; 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 Colenterates, or the re- 
