238 ANIMAL BIOLOGY. [Part II. 



down, but not to and fro ; between 2 and 3 there is little 

 motion; between 3 and 4 the motion is to and fro; between 

 4 and 5 up and down ; between 5 and 6 again to and fro ; and 

 between 6 and 7 once more up and down. The motions though 

 nearly, are, however, not quite at right angles ; so that the 

 motion of the limb, as a whole, is tolerably free. The way in 

 which the several divisions are hinged will be readily seen on 

 examination of the limb. Fig. 74, L, shows the chela of the 

 forceps from which the soft parts have been removed, and which 

 has been opened out by the removal of its dorsal wall, except 

 where it is dotted in the diagram. The motion on the hinge- 

 line (&.), at two opposite points on the exoskeleton, is in the 

 plane of the paper. At two points of the exoskeleton, at right 

 angles to the hinge-points, there project inwards into the cavity 

 of the chela two long flat processes. To one of these (ad.) 

 is, during life, attached the adductor muscle ; to the other and 

 smaller process (db.) is attached the abductor muscle. By the 

 former the jaws are closed ; by the latter they are opened. The 

 other joints of the limbs are similar in principle. 



The exoskeleton consists of a tough animal substance, chitin, 

 the product of the epithelial cells. Except where a joint renders 

 flexibility essential it is impregnated with calcareous salts (car- 

 bonate, with about one-eighth phosphate, of lime). The structure 

 of the exoskeleton (from the ophthalmite) is shown in Fig. 74, 

 K. There is an external wrinkled layer (epiostracum), below 

 which is a laminated mass in which are a vast number of close- 

 set wavy canals running outwards through its thickness. The 

 laminae are close-set externally (ectostracum, ec.), but wider 

 apart in the layers below (endostracum, en.). In the lowest 

 layers they again gradually become close-set and softer. A sec- 

 tion of the dried exoskeleton is easily made by grinding down 

 on a file or on pumice-stone, and polishing on a fine-grained hone. 



The Branchial Chamber. If we cut with a pair of scissors from 

 behind forwards along the branchiocardiac groove, and then out- 

 wards along the cervical groove, a flap of exoskeleton, the 

 branchiostegite, will be removed, and the plume-like branchiae 



