338 THE CANADIAN NATURALIST. [Sept. 



said to resemble the Insects in their flight, yet in their flittinga 

 to and fro they are not unlike ; and in their ceaseless, unwearying 

 crawlings the likeness holds good ; — as scavengers, too, they claim 

 brotherhood with a world of Beetles and other Insects. In this 

 however, as well as in the less amount of concentration of their 

 organs, they differ from Insects— namely, the changes which the 

 latter undergo are from one distinct stage to another, such as 

 caterpillar, chrysalis, butterfly ; but in the Crustacea we have 

 successive moultings of the crust, with some alteration in the 

 body, corresponding with the growth of the individual ; and 

 though these changes are often striking (in the young state of 

 Crabs, for instance),. yet there is no break in the line of life, no 

 dormant period, no transition from one mode of living to another, 

 as there is in Insects. 



However diversified the forms of the different kinds of Crustacea 

 may be— however varied the number and disposition of their 

 limbs, yet this great group have, with few exceptions, their 

 articulated framework as a feature in common ; and if that be 

 wanting, still (according to Huxley) the uniformly similar, six- 

 limbed, and Nauplius-like form in which so many members of the 

 lower groups of Crustacea begin their existence, furnishes a strong 

 connecting link among them. 



The diversity of organs among the Crustacea is almost endless; 

 what serves as jaws in one division are legs in another ; the 

 antennae in one may be organs of sense, in another of locomotion 

 or of prehension : then there are thoracic branchiae in some 

 /Decapods), sac-like branchial appendages in others (Tetradeca- 

 pods) ; whilst the Entomostraca rarely have any true branchiae, 

 the surface of either some part or of the whole of the body serving 

 for aeration. 



In the Crabs, which present the condition of highest centralisa- 

 tion for the Crustacea, the three front segmental elements are 

 coalesced and modified as the organs of feeling, sight, and hearing ; 

 the next six supply the mandibles, maxillae, and palpi for the 

 mouth; five are devoted to the organs of locomotion and prehen- 

 sion ; and the remainder are lost in the abbreviated abdomen or 

 tail-piece. In the other Decapoda (with ten limbs) also, such as 

 Lobsters, &c., nine segments and their pairs of appendages are 

 thus concentrated into the organs of sense and the mouth. In 

 the Tetradecapoda (with fourteen limbs), such as the Woodlouse, 

 &c., only seven segments are concentrated for these cephalic organs, 



