Lobster — A ppendoges . 
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 S 1-3 
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 (Eig. 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 the second, much larger, 
is the “ basis.” 
If the other limbs be compared with the swimmerets it will be 
found that they can be derived, without much difficulty, from the 
simple type. The antennules (Eig. 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 
