CRUSTACEANS. 
47 !) 
The Crustaceans have eyes of two kinds — simple and compound : 
the first are sessile and immovable, and very convex ; the other borne 
°n a short calcareous stem or peduncle, and formed of a number of 
small eyes symmetrically agglomerated — the reunion of all the micro- 
scopic cornea of a composite eye, resembling in shape a cap formed of 
facets. It is said, for instance, that the eye of the lobster consists 
°1 2500 of these little facets. The simple eyes are my opus, or 
short-sighted— the compound eyes for more distant but perfect sight. 
They appear to have a strong sense of smell. Many of them cannot 
swim, but walk with more or less facility at the bottom of the water. 
It is said, for instance, that the cavalier of the Syrian coast, Oxxjpoda 
cursor (Fabricius), is named from the rapidity with which it traverses 
great distances ; but it may be doubted if its pace equals that of a 
horse. 
Many systems have been proposed by different writers for the 
arrangement of the Crustacea. That proposed by Mr. Milne Edwards 
recommends itself, being founded on anatomical examination and 
a ctual experiment made by himself and M. Audouin. He divides 
them into two great divisions I. Those in which the mouth is 
furnished with a certain number of organs adapted for the prehension 
°r division of food. II. Those in which the mouth is surrounded by 
ambulatory extremities, the bases of which perform the part of jaws. 
The first includes the Maxilosa or Mandibulata, again divided 
mto Decapoda, having branchiae attached to the sides of the thorax, 
a nd enclosed in special cavities. The Decapoda are divided into : 
I- Bbaohtoba, namely, the Crabs. Cancer , Portumnus, Qrapsus, 
® cypodes , and Dorypes, belong to this group. 2. Axomoura, 
^eluding Dronria, Pagurus , Porcellana , and Hippa. 3. Macroura, 
deluding the Lobsters, Astacus, Pahomov, , the Craw-fish, Pcdinurus. 
Stomapoda, with external branchiae, sometimes rudimentary, some- 
times none. Thoracic extremities prehensile, or for swimming generally, 
‘ Slx or eight pairs. This division includes the Mysidee, Phyllosoma, 
^fiuilla, &c. 
The other divisions of Crustaceans we need not dwell upon. They 
are very obscure, and little known ; some of them fresh-water Crus- 
taceans, as in the Entomostraca ; others are parasitic on the whale 
and various species of fish. 
