CRUSTACEA. 143 



Gen. 10. HIRUDO, Lin. Lam. 



Body oblong, blunt, slightly depressed, widening posteriorly, 

 composed of numerous contractile segments, and with the 

 posterior extremity terminated by a broad prehensile disc ; 

 mouth naked, dilatable, armed interiorly with three teeth or 

 horny jaws ; no eyes ; anus superior, near the posterior disc. 



H. medicinalis, Lin. The Medicinal Leech. Body elongated, black- 

 ish, with six yellow lines above, spotted beneath with yellow and 

 black. Inhabits Europe in marshes, ponds, and slow running 

 waters. B. Sup. Encyc. Brit. i. pi. 26. 

 The common Medicinal Leech is well known, [t is viviparous, and the ova are 



imbedded in a glatinous mass, enveloped in a strong semitransparent membrane. 



H. sanguisuga, Lin. The Horse Leech. Body elongated, black, 

 cinereous green below, with black spots. Inhabits Europe, in 

 ponds and ditches. B. Pen. Brit. Zool. iv. 70. 

 The Horse Leech, when preserved in water, forms a good barometer, predicting 



bad weather by its great restlessness. 



CLASS VI. CRUSTACEA. 



Invertebral Animals with a crustaceans and more or less solid 

 covering, provided with articulated members, distinct or- 

 gans of circulation, and respiring by branchice. 



THE animals of this Class were known to the Greeks under 

 the name of vuXaxoffrgaxos, as designating marine animals of 

 which the exterior envelope was much less solid than that of 

 the testaceous, and much more so than the covering of the nak- 

 ed Mollusca. Among the Romans this designation was signi- 

 fied by the terms Crustata and Crustacea, the last of which 

 forms the present name for the class. 



The earliest modern naturalists, like the more ancient wri- 

 ters, arranged the Crustacea between the fishes and the Mol- 

 lusca; and Linnaeus placed them in his class Insecta, along with 

 the apterous insects, including the spiders in the same class. 

 Brisson was the first who formed them into a separate group, 

 though since his time Latreille and Cuvier, in their earlier works, 

 followed Linnaeus in classing the Crustacea with the Insects. 

 1VL Lamarck, however, adopted the division of Brisson, and also 

 formed a separate class of the ARACHXIDES, an arrangement 

 which has been followed by subsequent zoologists. 



