166 LECTURE XIV. 



of the segments of the thorax, and abdomen. Certain genera, as Se- 

 rolls and Bopyrus^ have the tergal arcs of the segments trilobed ; but 

 they exceed not the characteristic number in the Malacostraca, and 

 the seven rings of the thorax are clearly indicated in each by the seven 

 pairs of articulated feet which they support, although these are very 

 small in the parasitic Bopyriis. In the Cymothoa {Jig. 89.) the seven 

 thoracic and seven abdominal segments are more distinctly charac- 

 terised. 



The seven segments of the head are rather indicated by the ap- 

 pendages of that part than demonstrable in any of the Crustacea. 

 The Stomapoda afford, in the genus Squilla, the most favourable ex- 

 amples for studying the conformation of the head. The first segment 

 supports the pedunculated eyes : the second the smaller antennae : 

 the third and fourth segments are confluent, but indicated by the 

 larger pair of antennae and a pair of mandibles ; the tergal part of these 

 confluent segments is greatly developed, and extends over the rest 

 of the head and part of the thorax. Three other pairs of jaws indicate 

 the rest of the seven cephalic segments ; and these are succeeded by 

 the seven thoracic rings and their articulated appendages.. Of these 

 the first two pairs, which are organised for locomotion in the isopods 

 and amphipods, are now modified to serve as jaws ; and the remain- 

 ing five pairs are reserved for locomotion in all the podophthalmous 

 Crustacea. The first of these five ambulatory thoracic legs is com- 

 monly the largest, and didactyle. They all consist of six joints, and 

 have usually two appendages attached to their base, called the palp 

 and flagellum : analogous appendages are attached to most of the tho- 

 racic and cephalic articulated appendages, which subserve as jaws. 



The tergal arc of the third and fourth cephalic segments extends 

 over all the thoracic segments in the macrourous and brachyurous 

 Crustacea, and constitutes the broad carapace in the crab. In most 

 Macroura the thoracic shield is formed of the lateral or epimeral ele- 

 ments of the fourth cephalic ring, which meet along the back, and 

 give way preparatory to the moult. The tergal elements of the tho- 

 racic rings are not developed in either crabs or lobsters ; when these 

 rings are exposed by lifting up the cephalo-thoracic shield, the epi- 

 meral parts alone are seen converging obliquely towards one another 

 but not joined at their apices. The thoracic legs, besides serving for 

 mastication, prehension, and locomotion, usually support more or 

 less of the branchial apparatus, and certain pairs are perforated by 

 the generative ducts, except in certain crabs, in which the sternal 

 arc, which is of unusual breadth, supports the generative outlets. 



The external arcs of the thoracic segments send inwards certain 

 processes, called apodemata, which include spaces for protecting the 



