ENTOMOSTRACA. 647 



three appendages terminating in four spines, whence the spe- 

 cific name. These are of a dark horn color, and frequently 

 found hidden under stones and sea-weed, 



The genus Gammarus includes FRESH-WATER SHRIMPS, which 

 are very active, and common in running streams. They may 

 often be found under stones and pieces of wood. These have 

 the last joint of the antennae composed of numerous minute ones ; 

 the upper antennae are as long as the lower, and sometimes 

 longer, with four articulations, the last ending in a bristle, the 

 lower antennae have five articulations ; the tail has small, bundle- 

 like spines. 



We can barely na*me the parasitic Cymothoids, (genus Cymo. 

 thoa,) which are fourteen footed, formerly arranged with the 

 Isopoda, most of which attach themselves to the mouths and 

 gills of fishes, and of which the Seriolis has been thought to 

 present, at first sight, a resemblance to the extinct form of the 

 Trilobites. 



Ligia is another genus of the present order, having an oval, 

 oblong body, with transverse segments, and two short append- 

 ages at the end of the tail. The two outer antennae are quite 

 conspicuous. (See figure of L. oceanica, Plate XVI. fig. 2b.) 



Limnoria is another marine genus, which has the head nearly 

 as large as the first segment; the tail has six distinct rings 

 with two appendages on each side. The species L. terebrans, 

 (Lat. boring,) can roll themselves up into a ball. These, and 

 the Ligia oceanica, both in great numbers sometimes attack the 

 timbers of ships, docks, etc., and soon render them useless. 



THIRD ORDER. ENTOMOSTRACA, (Gr. evTopa, entoma, an insect; 

 , ostrakon, a shell, i. e., shell insects.) 



This term is applied to Crustaceans for the most part inhabit- 

 ing fresh-water. In these, the nervous knots which supply the 

 place of the brain, consist of one or two globules merely. The 

 heart assumes the form of a long vessel. The gills are com- 

 posed of hair-like processes, forming a portion of the feet, or of a 

 certain number among them, and sometimes the mandibles and 

 upper jaws. The number of feet varies, and in some genera is 

 said to be over a hundred. Nearly all have a shell, consisting 

 of one or two pieces, generally almost membranous and 

 transparent, the coverings are like those of the insects, rather 

 horny than calcareous. The antennae, varying much in form 

 and number, serve in many species for swimming. 



Dr. Baird says, most of them are "essentially carnivorous." 



