THE CRUSTACEA OF NEW JERSEY. 39 



prostomiunii tO' form head, while others mostly forming two sec- 

 tions, as thorax and abdomen. Thoracic seg^ments may be fused 

 in greater or less number to form a cephalothorax. Head may 

 have median eye, this often disappearing in adult life, and pair 

 of compound eyes, both belonging to prostomial region. In case 

 O'f latter jointed and elevated eye-stalks frequently developed. 

 Head with appendages usually in five pairs. First, or anten- 

 nules usually considered as of first metamere, more likely pros- 

 tomial. Second, or antennae, are postoral or metameric ap- 

 pendages mjoved forwards to praeoral position. Third, are 

 mandibles or crushing-jaws. Fourth are first maxillce. Fifth 

 are second m|axill?e. Thoracic and abdominal appendages of 

 variable modification, as jaws, legs, fins or accessory reproduc- 

 tive organs. Except antennules, appendages typically biramous, 

 formed of a stem or protopodite which bears two branches, as 

 endopodite and exopodite. External covering of body a chitinous 

 cuticle, becoming thick and somewhat calcified in places where no 

 movement is necessary, forms series of hard parts of sclerites, 

 separated by flexible chitin, so that whole chitinous cuticle thus 

 formis exoskeleton. Typically one sclerite to each metamere be- 

 hind head and to- each podomere in appendages, but concrescence 

 of sclerites often obtains. Exoskeleton produced into set^e, 

 which are hollow processes of cuticle containing prolongations of 

 underlying epidermis. Respiration either by general surface of 

 body or by gills, which are hollow offshoots of thoracic wall or 

 of thoracic or abdominal limbs. Stemodseumi and proctodeum 

 formi considerable portion of enteric canal, lined with chitin, and 

 mesenteron gives rise to digestive glands. Body-cavity divided 

 into compartments, most of which contain blood and are por- 

 tions of vascular system, though true ccelomie may be represented 

 by coinpartments of body-cavity not containing blood and by 

 cavities of reproductive organs. A vascular system^ fonned of 

 contractile heart, as muscular dilatation of dorsal vessel and 

 communicating by valvular ostia with an enclosing pericardial 

 sinus. Blood taken from heart to various organs by arteries, 

 returned to- pericardial sinus by sinuses and veins, and respira- 

 tory organs interposed in returning current. Renal organs i>ecu.- 



