300 



LECTURE XIV. 



third segment is variously termed, contains fifteen fettered segments 

 in the Asaphus caudatus : the segments of both thorax and abdo- 

 men are very similar to each other, and gradually decrease in size. 

 They are divided by two longitudinal furrows into three lobes. The 

 head usually supports a pair of large compound eyes situated near the 

 sides, like the larger outer pair of eyes in the Limulus, which they 

 resemble in form and structure. 



The Malacostraca are divided into two groups, according to the 

 attachment of the eyes : those with immoveable sessile eyes form the 

 Edriophthahna, those with moveable pedunculated eyes the Podoph- 

 ihalma. 



The lower organised or edriophthalmous forms of malacostracous 

 Crustacea resemble the Trilobites in the non-confluence and uniformity 

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

 rolis 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 Bopyrus. In the Cymothoa {Jig. 129) the 

 seven thoi'acic and seven abdominal segments are more distinctly 

 characterised. 



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 antennas 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 seg- 

 ments ; and these are succeeded by 

 the seven thoracic rings and their 

 articulated ajjpendages {fig. 128, a). 

 Of these the first two pairs, which 

 are organised for locomotion in the 

 isopods and amphipods, are now mo- 

 dified to serve as jaws ; and the re- 

 maining five pairs (ib. 3 — 7) are 



reserved for locomotion in all the Stemal arcs of thorax, Astacus. 



podophthalmous Crustacea. In the Decapoda (Crabs, Lobsters, 



130 



