126 ZOOLOGY. 



Class ni.— ECHIXOmEA. 



Sphetncal, heart-shaped, or disk-Uke EcMrwderms, with a solid sheU ofim- 

 moxable plates, bearing interambulaci-al spines; with a mouth and anaZ 

 opemng, the mouth in most of tlie species armed with fide teeth; am- 

 bulaeraZfeet irell developed. The sexes distinct. Bi ctlopment either direct, 

 or, as in most cases, by a marked metaTnorphosis from a pluteus larta. 



Order 1. Palecltinida. — Shell composed of more than tsveuty rows of 

 plates. Suborder 1. ildonitida fMelonites, Protechinus, 

 Paleechinus, Archaeocidaris). Suborder 2. Eoddaria (Eoci- 

 daris). 



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

 Suborder 1. Desmostic/.a (Cidaris, Echinus, Strongylocen- 

 trotus, Echinometra, Clypeaster, and Echinarachnius). 

 Suborder 3. 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- 

 aiuded of the spines in order to study the relations of the ambulacral, 

 interambulacral, and genital plates. 



Class JX. — Holothueoidea Sea-cucumhers]. 



Creneral Cliaracters of Holothurians.— We now come to 

 Eehinoderms in which the body is usually long, cylin- 

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

 tain genera, as Sijnapta, Chirodota, and Evpyrgns, 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 

 1)6611 led to the adoption of one of two views : first, either 

 that the worms and Eehinoderms have had a common origin, 

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

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



