190 



Arthropoda. 



hind part of tlie head, and covering a greater or less part of the body. 

 It is frequently concrescent with the thorax along the mid-dorsal line. 

 Sometimes the mid-line of the shell is softer than the rest, so that it is 

 divided into two movable halves, which surround the body like a 



Pig. 151. Limbs of different Crustacea ; the exopod is dotted. .4 swimming leg of 

 Branchipus. B swimming leg ofaCopepod. second maxilla of a Decapod. 

 D mandible of a Copepod. s endopod, 1, 2, 3 its joints, p palp, ep epipod. — Orig. 



lamellibranch shell. The outer surface of the shell is usually covered 

 with a thick, hard cuticle, so that it forms a really protective covering ;, 

 the inner surface, is, on the other hand, softer. The carapace is very 

 characteristic of the Crustacea, although it is absent from many forms. 



The cuticle covering the body is often of a considerable thick- 

 ness and hardness, the chitin always containing lime salts (carbonate 

 of lime) in varying quantity. 



The olfactory- organs are situated on the antennules, the 

 function appearing to be restricted to long, soft, thread-like setse. 

 Auditory organs are known only in some Malacostraca (see 

 Mysidse, Decapoda) . Optic organs may be represented either by 

 the nauplius-eye, consisting of a single eye-spot in the median 

 line of the head, or of a small group of them, or by a pair of large- 

 compound eyes on the sides of the head, often placed upon movable 

 or immovable stalks. In some forms both median and paired eyes are 

 present, in some only the former, in many only the latter. In numerous, 

 cases the nauplius-eye is present in the larva, but atrophies later. 



The digestive tract begins anteriorly on the ventral surface 

 of the head, in a mouth opening between the mandibles. It is often 

 bounded before and behind by projecting folds of skin, the upper, 

 or the under lip. The alimentary canal is a straight tube, which 

 opens upon the terminal segment of the body. A liver is usually 

 present. 



Respiratory organs. The Crustacea, for the most part, 

 breathe the oxygen dissolved in water. In many, particularly in 

 small thin-skinned forms, special respiratory organs are wanting ; 

 then the whole surface of the body, or the greater part of it, performs 



