42 E-UPERT Jones, on Bivalved Entomostraca. 



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 disjDosi- 

 tion of their limbs, yet this great group have, with few 

 exceptions, their articulated framework as a feature in 

 common j 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 j^rehension : then there are 

 thoracic branchiae in some (Decapods), sac-like branchial 

 appendages in others (Tctradecapods) ; whilst the Ento- 

 mostraca rarely have any true branchiae, the surface of either 

 some part or of the whole of the body serving for aei'ation. 



In the Crabs, which present the condition of highest 

 centralisation 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, 

 maxillee, and palpi for the mouth ; five are devoted to the 

 organs of locomotion and ^^rehension ; and the remainder are 

 lost in the abbreviated abdomen or tail-piece. In the other 

 Decapoda (with ten limbs) also, such as Lobsters, «S:c., nine 

 segments and their pairs of appendages are thus concentrated 

 into the organs of sense and the mouth. In the Tetradeca- 

 poda (with fourteen limbs), such as the Woodlouse, &c., only 

 seven segments are concentrated for these cephalic organs. In 

 the Entomostraca, only six thus coalesce for the senses and 

 mouth in the Cyclops group, only five in the Daphnia and 

 CaJigus, and only /owr in Limulus. 



The essential points in the framework of the body of an 

 Entomostracan of low organization, and in the arrangement 

 of the organs, are avcII seen in the Brine-shrimp {Artemia). 

 Here the body has numerous articulations or seginented jjor- 

 tions. The head-part takes ujd four or five coalesced somites, 

 bearing the antenna?, eyes, and masticatory organs ; eleven 

 pairs of natatory and branchial limbs follow on eleven seg- 

 ments ; the next two joints or rings have their own modified 

 appendages; seven segments succeed, without appendages. 



