[Illustration: FIG. 3.—_Astacus fluviatilis._—Ventral or sternal views (nat. size). A, male; B, female:—_a_, vent; _gg_, opening of the green gland; _lb_, labrum; _mt_, metastoma or lower lip; _od_, opening of the oviduct; _vd_, that of the vas deferens. _1_, eye-stalk; _2_, antennule; _3_, antenna; _4_, mandible; _8_, second maxillipede; _9_, third or external maxillipede; _10_, forceps; _11_, first leg; _14_, fourth leg; _15, 16, 19, 20_, first, second, fifth, and sixth abdominal appendages; X., XI., XIV., sterna of the fourth, fifth, and eighth thoracic somite; XVI., sternum of the second abdominal somite. In the male, the 9th to the 14th and the 16th to the 19th appendages are removed on the animal’s left side: in the female, the antenna (with the exception of its basal joint) and the 5th to the 14th appendages on the animal’s right are removed; the eggs also are shown attached to the swimmerets of the left side of the body.] The four pairs of legs which are employed for walking purposes, are divided into a number of joints, and the foremost two pairs are terminated by double claws, arranged so as to form a pincer, whence they are said to {22} be _chelate_. The two hindermost pairs, on the other hand, end in simple claws. In front of these legs, come the great prehensile limbs (_10_), which are chelate, like those which immediately follow them, but vastly larger. They often receive the special name of _chelæ_; and the large terminal joints are called the “hand.” We shall escape confusion if we call these limbs the _forceps_, and restrict the name of _chela_ to the two terminal joints. All the limbs hitherto mentioned subserve locomotion and prehension in various degrees. The crayfish swims by the help of its abdomen, and the hinder pairs of abdominal limbs; walks by means of the four hinder pairs of thoracic limbs; lays hold of anything to fix itself, or to assist in climbing, by the two chelate anterior pairs of these limbs, which are also employed in tearing the food seized by the forceps and conveying it to the mouth; while it seizes its prey and defends itself with the forceps. The part which each of these limbs plays is termed its _function_ and it is said to be the _organ_ of that function; so that all these limbs may be said to be organs of the functions of locomotion, of offence and defence.