CRUSTACEA. 



we have in fact a repetition of the condition of the skeleton 

 met with in the Myriapoda, or in the larva state of many in- 

 sects ; the whole body being composed of a series of similar 

 segments, to which are appended external articulated members of 

 the simplest construction (Jig. 149). 



The number of rings or segments composing the body varies 

 in different species ; but such variation would seem, from the inter- 

 esting researches of Milne Edwards and Audouin, concerning the 

 real organization of articulated tegumentary skeletons, to be rather 

 apparent than real, inasmuch as the discoveries of these distinguished 

 naturalists go far to prove that, whatever the state of consolidation 

 in which the integument is found, the same number of elements 

 or rings may be proved to have originally existed before, by their 

 union, they became no longer distinguishable as separate seg- 

 ments. 



The normal number of these elements Milne Edwards considers 

 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 ab- 

 dominal region of the body. 



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 effected. 



In Talitra (Jig. 150) the cephalic elements are completely 

 united, their existence being Fi S- 15 * 



only indicated by the several 

 pairs of appendages ; one pair, 

 of course, belonging to each 

 ring. The first ring of the 

 cephalic region, in this in- 

 stance, has no external articu- 

 lated member ; but in higher 



orders the eyes are supported upon long peduncles connected with 

 this element of the skeleton, that may be regarded as the represen- 

 tatives of those limbs which take different names in different regions 

 of the body. The second and third rings support jointed organs 

 here called antennae ; while the several pairs of jaws appertaining to 

 the mouth indicate the existence of so many elements united toge- 

 ther in the composition of the head. 



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, 



