434 CRUSTACEA. 



holes in walls, under stones, &c. They feed upon decaying vegetable and animal matter, and only come forth 

 from their retreat in wet and moist weather. They crawl but slowly, at least, when not alarmed. The eggs are 

 inclosed in a pectoral pouch. Tlie young, when first hatched, are destitute of one of the thoracic 

 segments, and consequently of a pair of legs, which they subsequently acquire. They were formerly 

 much used in medicine, but their employment has long been discontinued. (Types, Oniscus mu- 

 rarius. Fab. ; Cloporte ordinaire, Geoff. ; Cloporte asclle, De Geer.) 



Porcellio, Latr., differs from Oniscus in having only seven joints in the lateral antennae. (Oniscus 

 asellus, Cuv.) 



yirmarfi'Wo, Latr., differs from all the preceding in the posterior appendages of the body not being 

 exserted. The last segment is triangular. Tlie lateral antennae have only seven joints, the upper sub- 

 abdominal plates have a row of small apertures. {Oniscus armadillo, Linn. ; O. cinereus, Panzer — 

 Armadillo pustulatus, Desm. — Armadillo officinalis, Dumeril, from Italy, a species fonnerly much 

 employed by the apothecaries.) 



SECOND GENERAL DIVISION. 



CRUSTACEA ENTOMOSTRACA (MiiUer). 



Under this denomination, formed from the Greek, and signifying insects in a shell, 

 Otho Frederick Miiller comprised the genus Monoculus of Linnaeus, to which some of 

 his Lernsese must also be added. The researches of MiiUer upon these animals, of 

 which the investigation is rendered the more difficult owing to their general micro- 

 scopical size, together with those of SchafFer and the elder Jurine, have excited the 

 admiration, and merit the thanks, of all naturalists. Other works, but of a more par- 

 tial nature, as those of Ramdohr, Strauss, the younger Hermann, the younger Jurine, 

 Adolphe Brongniart, Victor Audouin, and Milne Edwards, [to which we may add the 

 more recent memoirs of Dr. Loven in Sweden, of Dr. Johnston and William Baird in 

 our own country, and of Dana in America] , have greatly extended our acquaintance 

 with these animals, especially in respect to their anatomy. M. Strauss far surpasses 

 the others, although forestalled, as well as the elder Jurine, in various important struc- 

 tural observations, by Ramdohr, whose memoir upon Monoculus, published in 1805, 

 appears to have been unknown to those authors. Fabricius contents himself with 

 adopting the genus Limulus of Midler, which he places in his class Kleistagnatha, or 

 our Brachyurous Decapoda. All the rest of the Entomostraca he reunites, after 

 Linnaeus, in a single genus Monoculus, placed in his class Polygonata, or our Isopo- 

 dous Edriopthalma. 



All these animals are aquatic, and ordinarily inhabit fresh water. Their legs, of 

 which the number is variable — reaching, in some species, to beyond a hundred — are 

 generally fitted only for swimming, and are sometimes ramified or divided, sometimes 

 ornamented with long feathered hairs, or composed of plate-like joints. Their nervous 

 system is composed of only one or two globules. The heart has also the form of a 

 long vessel. Their branchiae, composed of hairs or threads, either isolated or united, 

 so as to form beards, combs, or tufts, form part of the legs, or at least of a certain 

 number of them, as well as, occasionally, of the mandibles and upper maxillae. (See 

 Cypris.) Hence the origin of the name Branchiopoda, which we applied to these ani- 

 mals, and which we at first united into a single order. 



Nearly all the species have a shell of one or two pieces, of very slender consistence, 

 and generally nearly membranous and almost diaphanous, or at least they have a large 

 anterior thoracic segment, often soldered with the head, and appearing to occupy the 



