346 
POPULAR SCIENCE REYIEW. 
probably answer more to the deeper part (or true dermis) of our 
own skin. 
Now, in order to understand this external skeleton, let us 
begin with the simplest structure, and take one of the joints 
(or somites) of the abdomen — the second from the head and 
fore-part of the body, Le, from the cephalo-thorax. It consists 
of a curvex upper part (tergum) and a flatter inferior side 
{sternum)', the angle of junction on each side of the tergum 
and sternum is produced downwards, and is termed the pleuron. 
The sternum bears the shallow sockets, each of which gives 
attachment to one of the swimmerets. Each swimmeret is made 
up of the single joint {protopodite, or root-footlet) which joins 
the sternum, and of two other equal-sized joints which hang from 
the single joint. The external one of the pair is termed the 
exopodite, or outer footlet ; the internal one is called the endo~ 
podite, or inner footlet. If we examine the other somites of the 
abdomen, successively backwards, we shall find they all closely 
resemble the one described, except the last somite, and that differs 
(1) in having attached to it the terminal median piece before 
spoken of, the telson, which is merely a process and no somite ; 
and (2) in having its swimmerets greatly broadened out, and each 
exopodite divided into two by a transverse joint. These broad 
swimmerets, together with the telson, form the expanded ter- 
mination of the abdomen, which, by its forward projection 
through the water, drives the animal backwards. Thus we see 
that so far the lobster’s body consists of a longitudinal series of 
parts which, in a certain sense, may be called ‘‘ the same,” the 
similar component parts being repeated in each ; and we see that 
modifications in size or shape in even a process of segmentation 
does not destroy this sameness,” the divided exopodite of the 
terminal somite answering to the undivided exopodite of any 
other abdominal somite. . 
With the most anterior of these abdominal somites, however, 
we find a further change, the appendages which answer to the 
swimmerets of the other somites being here modified in the 
males into a pair of grooved processes, each like a marrow- 
spoon; in the female, into flexible, soft processes. 
We come now to the cephalo-thorax, which at first seems to 
afford no sign of an essentially similar construction, a sort of 
furrow on the dorsal surface of the shell {or carapace) alone 
marking off an anterior portion, the head, from the rest of the 
complex structure. 
If the ventral or sternal surface, however, be looked at, we 
then can detect evidence that the cephalo-thorax really consists 
of a number of somites fused, as it were, into one, and each 
somite has fortunately preserved its pair of appendages, different 
as we shall find these to be both from each other and from the 
wimmerets of the abdomen, 
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