562 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 ; (2) the antennae, 

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

 wards to a pre-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 sclcritcs, 

 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 podomere in the append- 

 ages, but concrescence of sclerites frequently takes place. The 

 exoskeleton is produced into seta?, 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 digestive glands. 

 The body-cavity is divided into comparments, 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 which nearly always includes a con- 

 tractile 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 respiratory 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 maxillae, or of antennary (green) glands opening on the* 

 antennae. 



The nervous system consists of a brain united by oesophageal 

 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 either amoeboid with 



