XIPHOSUEA. 



481 



Se- 



armour and is divided into an arched cephnlo-thorax and a flat, 

 almost hexagonal abdomen, which ends in a movable sword-like 

 caudal spine. The cephalo-thorax (fig. 374) forms by far the larger 

 part of the body; it bears on its arched dorsal surface two large 

 compound eyes, and further forwards, nearer the middle line, two 

 smaller simple eyes ; while on its ventral surface there are six pairs 

 of appendages, of which the anterior pair 

 is slender and may, on account of its 

 position in front of the mouth, be re- 

 garded as a pair of antennae, although it 

 ends, like the others, with a chela. The 

 latter are placed to the right and left 

 of the mouth, and their coxal joints serve 

 as organs for the mastication of the food. 

 At the end of the cephalo-thorax there 

 is a pair of lamellar appendages, which 

 are connected in the middle line and form 

 a kind of operculurn for the branchial ap- 

 pendages of the abdomen. It seems of 

 interest that the form of this branchial 

 operculum in the Asiatic and American 

 species presents constant differences, in 

 that the median portion in the former is 

 undivided, and hi the latter consists of 

 two joints. The shield-shaped abdomen 

 which, by means of a transverse joint, is 

 movable on the cephalic shield in a dorso- 

 ventral direction, is armed on either side 

 with movable spines, and bears on its ven- 

 tral surface five pairs of lamellar feet, 

 which are almost completely covered by 

 the operculum. These abdominal feet 

 assist both in swimming and in respira- 

 tion, since the respiratory lamellaa are 

 placed on them (fig. 374, a, b). 



The internal organization attains a re- 

 latively high development in correspondence 

 with the large size of the body. In the 



( nervous system the following parts can be distinguished : a broad 

 Desophageal ring, the anterior part of which constitutes the brain 

 ind gives off the optic nerves, while from the lateral parts the 



31 



FiG. 374. a Limit? ut moluccanus, 

 seen from the dorsal side 

 (after Huxley). O, eyes; St, 

 caudal spine. I, L. rotitndi- 

 cauda (after M. Edwards), 

 seen from the ventral side. A 

 Antennae; B, the feet with 

 their coxal jaws ; K, gills ; Op, 

 operculum. 



