CLASS CRUSTACEA. 137 



number of moultings. With the exception of a small num- 

 ber, in which the changes of the skin exercise a trifling influ- 

 ence on their primitive form, modify or augment their locomo- 

 tive organs, these animals are, when born, size excepted, such 

 as they will remain for the whole of their existence. 



DIVISION OF THE CRUSTACEA INTO ORDERS. 



The situation and form of the gills, the manner in which 

 the head is articulated with the trunk, the mobility or fixed- 

 ness of the eyes, the masticatory organs, and the teguments, 

 will form the basis of our divisions, and give rise to the fol- 

 lowing orders : — 



We shall divide this class into two sections, the Malacos- 

 TRACA and the Entomostraca. The first have generally 

 very solid teguments, of a calcareous nature, and ten or four- 

 teen feet, usually unguiculated ; the mouth, situated in the 

 usual way, is composed of a tongue, a labrum, two mandibles 

 (often bearing a palpus), two pairs of jaws covered by the 

 jaw-feet; in a great number, the eyes are carried on an arti- 

 culate and mobile pedicle, and the gills are concealed under 

 the lateral edges of the testa or carapace ; in the others, they 

 are usually placed under the post-abdomen. This section is 

 composed of five orders, the Decapods, the Stomapods, 

 the L^modipods, the Amphipods, and the Isapods ; the 

 first four embrace the genus Cancer of Linnseus, and the last 

 that which he names Oniscus. 



The Entomostraca, or insects with shells, of Muller, com- 

 pose the genus Monoculus of Linnteus. Here the teguments 

 are corneous and very thin, and a testa in the form of a buck- 

 ler, of one or two pieces, or in the form of a bivalve shell, 

 covers or encases the body of the great majority ; the eyes are 

 almost always sessile, that is fixed, and often there is but one ; 

 the feet, the number of which varies, are in the majority ex- 

 clusively adapted for swimming, and without any claw at the 



