Lobster — Appendages. 15 



is seen to arise, as a fold of the skin, from the hinder. edge of Wall- 



the head-region, and to envelop the distinctly segmented thorax nos'^I-S 



like a loose jacket. In the Lobster, this fold has coalesced, down 



the middle of the back, with the terga of the thoracic somites, but 



at the sides it hangs free, enclosing a " branchial cavity " in which 



the gills lie between it and the side of the body. The free part 



of the carapace which covers the branchial cavity is known as the 



" branchiostegite," and its front end is marked off on the outside 



of the carapace by an oblique " cervical groove " (Fig. 1), which 



has been supposed to indicate the limit between the head and the 



thorax. 



Appendages.— Excluding the movable stalks on which the 

 eyes are set and of which the nature will be discussed later, the 

 body of the Lobster carries nineteen pairs of appendages. In front 

 of the head are two pairs of feelers, the " antennules " and 

 "antennae" respectively (sometimes called the first and second 

 antennae) ; near the mouth are three pairs of jaw-appendages, the 

 strong "mandibles" and the flattened leaf-like "maxillulae" and 

 " maxillae " ; following these, which belong to the head-region, are 

 three pairs of thoracic appendages, the " maxillipeds," which form 

 a transition between the true jaws and the legs. The large claws 

 and the four pairs of walking legs may simply be termed " legs," 

 and together with the three pairs of maxillipeds, correspond with 

 the eight somites of the thorax already referred to. The six 

 somites of the abdomen have each a pair of appendages, those of 

 the first five being known as swimmerets (" pleopods "), while 

 those of the last somite are known as the " uropods," and are large, 

 flattened appendages spread out on each side of the telson to form 

 the tail-fan. All these appendages can be shown to be constructed 

 on a common plan, which is seen in a simple form in the case of 

 the swimmerets. Each of these consists (Fig. 2) of a stalk, the 

 " protopodite," with two branches known respectively as the 

 " endopodite " (on the inner side) and the " exopodite " (on the 

 outer side). The protopodite itself is composed of two segments ; 

 the first, very small, is the " coxa," and tlie second, much larger, 

 is the " basis." 



If the other limbs be compared with the swimmerets it will be 

 found that they can l)e derived, without much difficulty, from the 

 simple type. The antennuhs (Fig. 1), which appear most simple, 

 are perhaps the least easy to interpret. Although they plainly 

 consist, like the swimmerets, of a stalk and two branches, there are 

 reasons for doubting whether these three parts correspond with 



