OR OSSEOUS SYSTEM. ',W 



in a single transverse row. The long jointed palpi, at- 

 tached to the maxillae are seen at c, c, extended like 

 feet. In the four pairs of legs attached to the sides of 

 the cephalothorax, we observe the long femur /, extending 

 as in insects, from the trochanter e ; but we commonly 

 lind a smaller articulation or protibia interposed between 

 the femur and the tibial joint. In some there are only 

 six legs, as in insects. On the lower and anterior por- 

 tion of the abdomen, or sometimes on the posterior and 

 lower part of the cephalothorax, are placed the open- 

 ings into the; respiratory organs, which spiracula are 

 found to vary in number from two to eight, they 

 sometimes lead to pectinated simple air-sacs, and some- 

 times they open into extended and ramified tracheae. 

 Although these animals retain the form with which 

 they escaped from the ovum, they throw off periodically 

 their exterior covering, like the larvae of insects ; and like 

 the Crustacea, they reproduce entire legs when they have 

 been removed from the body. The last abdominal seg- 

 ment of the scorpion is in form of a sting, or ossified 

 tubular poison-duct, as we frequently see that segment in 

 insects. 



XIII. Crustacea. The crustaceous animals possess the 

 most solid form of the skeleton met with in the arti- 

 culated classes. It is found in the larger decapods to 

 contain nearly half its weight of carbonate of lime, and 

 there is also a considerable proportion of phosphate of 

 lime, with traces of magnesia, iron, and soda. These sub- 

 stances are exuded from the surface of the true skin, 

 along with a tough coagulable animal gluten, which con- 

 nects all their particles, and forms a thin varnish on the 

 surface. The colouring matter is generally beneath this 

 varnish, and on the exterior surface of the calcareous de- 

 posit, but sometimes it pervades the whole substance of 

 the shell. This extravascular crust forms hollow rings 

 to envelope the trunk, and tubular sheaths to cover ah* its 

 appendices, and these are periodically cast off and renewed 

 on the whole exterior of the body, to allow of the necessary 

 growth of the soft parts. The head and thorax are here 

 commonly united to form, as in arachnida, a cephalo- 

 thorax, which is covered above by a large continuous 



