520 ZOOLOGY SECT. 



median eye, which frequently disappears in the adult, and a pair 

 of compound eyes, both belonging to the prostomial region : the 

 latter frequently become elevated on jointed eye-stalks. The 

 appendages of the head are (1) the antennules, which are usually 

 considered as belonging to the first metamere, but are perhaps 

 more correctly to be looked upon as prostomial ; (2) the antennae, 

 which are certainly post-oral or metameric appendages shifted for- 

 wards to a prae-oral position; (3) the mandibles or crushing jaws ; (4) 

 the first maxillae ; and (5) the second maxillae. The thoracic and 

 abdominal appendages are variously modified as jaws, legs, fins, or 

 accessory reproductive organs. With the exception of the anten- 

 nules, the appendages are typically biramous, consisting of a stem 

 or protopodite bearing two branches, the endopodite and exopodite. 



The body is covered externally by a chitinous cuticle, which 

 becomes thickened and sometimes calcified in regions where no 

 movement is required, forming a series of hard parts or sclerites, 

 separated by flexible chitin : the whole chitinous cuticle thus 

 constitutes an exoskeleton. Typically there is one sclerite to each 

 metamere behind the head, and to each podomore in the append- 

 ages, but concrescence of sclerites frequently takes place. The 

 exoskeleton is produced into setae, which are hollow processes of 

 the cuticle, containing prolongations of the underlying epidermis. 



Respiration takes place either by the general surface of the 

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

 of the thoracic or abdominal limbs. The stomodaeum and proc- 

 todaeum form a considerable portion of the enteric canal, and are 

 lined with chitin : the mesenteron gives rise to digestiv^ glands. 

 The body-cavity is divided into compartments, most of which 

 contain blood and are portions of the vascular system : the true 

 coelome may be represented by compartments of the body-cavity 

 not containing blood and by the cavities of the reproductive organs. 

 There is a vascular system consisting of a contractile heart, formed 

 as a muscular dilatation of a dorsal vessel, and communicating by 

 valvular ostia with an enclosing pericardial sinus. The blood is 

 taken from the heart to the various organs by arteries, and is 

 returned to the pericardial sinus by sinuses and veins : the re- 

 spiratory organs are interposed in the returning current. The 

 renal organs are peculiarly modified nephridia, which may take 

 the form either of shell-glands opening on the second maxilla, or 

 of antennary (green) glands opening on the antenna. 



The nervous system consists of a brain united by cesophageal 

 connectives with a ventral nerve-cord, formed of a double chain 

 of ganglia joined together by commissures and connectives. The 

 first three pairs of embryonic ganglia commonly unite to form 

 the brain, which is therefore a syn-cerebrum. The sexes are 

 separate or united : sexual dimorphism is common : partheno- 

 genesis frequently occurs. The sperms are usually non-motile : 



