EXTERNAL SKELETON OF CRUSTACEA. 387 



siders to be twenty-one, seven of which enter into the composition of the 

 head, seven belong to the thorax, and as many appertain to the abdo- 

 minal region of the body. 



(987.) To illustrate this important doctrine let us select a few 

 examples, in order to show the manner in which the progressive 

 coalescence of the segments is eifected. 



(988.) In Talitrus (fig. 196) the cephalic elements are completely 

 united, their existence being only j?j g jgg 



indicated by the several pairs of 

 appendages one pair, of course, 

 belonging to each ring. The first 

 ring of the cephalic region, in this 

 instance, has no external articu- 

 lated member; but in higher 

 orders the eyes are supported 

 upon long peduncles connected 



with this element of the skeleton, Talitrus. 



that may be regarded as the representatives of those limbs which take 

 different names in different regions of the body. The second and third 

 lings support jointed organs, here called antemice; while the several 

 pairs of jaws appertaining to the mouth indicate the existence of so 

 many elements united together in the composition of the head. 



(989.) The seven segments of the thorax are still distinct, and each 

 supports a pair of jointed organs, which, being used in locomotion, are 

 called legs ; the abdominal elements, likewise, are equally free, and have 

 natatory extremities developed from the five posterior rings. 



(990.) In the Lobster (Astacus marinus) we find not only the ce- 

 phalic segments anchylosed together, but those of the thorax also ; and 

 although the lines of demarcation between them are still recognizable 

 upon the ventral aspect of the body, superiorly the entire thorax and 

 head are consolidated into one great shield (cephaloihorax), the abdo- 

 minal segments only remaining distinct and moveable. 



(991.) In the Crabs the centralization of the external skeleton is 

 carried to still greater lengths, so as to enable this tribe of Crustaceans 

 to become more or less capable of leaving their native element and 

 walking upon the shores of the sea, or even, in some instances, of lead- 

 ing a terrestrial existence, as in the case of the Land-Crab of the "West 

 India Islands. The abdominal segments, however, still remain free, 

 though proportionately of very small dimensions ; and, being no longer 

 useful in swimming, the abdomen is folded beneath the enormously- 

 developed thoracic portion of the body. 



(992.) In the King-Crab (Limulus Polyphemus, fig. 197) even the 

 divisions of the abdomen are obliterated, the whole body being covered 

 by two enormous shields, and the tail prolonged into a formidable 

 serrated spine, of such density and sharpness that in the hands of 



2c2 



