INTEODTTCTION. 



XV 



is entirely wanting, and the flagellum is very small. The anten- 

 nules (Fig. iii., b) are also small, and are lodged, when retracted, 



rax. i. 



mx . 2. 



TXlCCp 2 



in n. 



mxpi 



Fig. IV. Oral appendages of a Crab (Leptodius). 

 mn, Mandible ; mx 1, first maxilla; mx 2, second maxilla; mxp 1, first 

 pair of maxillipedes ; mxp 2 second pair of maxillipedes. 



in distinct fossae excavated under the front, and separated from 

 one another by a vertical partition. The eyes are situated in 

 distinct orbits external to the antennae, but the base of the 

 peduncle is inserted, as in the Prawn, anterior and superior to 

 the insertion of the antennules. 



The gills of the Crab are fewer than those of the Prawn, 

 being at the most but nine in number on each side. They are 

 of the class of branchiae called by Huxley phyllobmnchice, i.e., 

 they consist of a central stem on which are set at right angles 

 two rows of thin plates or lamellae superimposed upon one 

 another like the leaves of a book. 



The circulatory, digestive, and reproductive organs of the 

 Crab are similar to those of the Prawn.* The nervous system 

 is much more concentrated the thoracic ganglia having coalesced 

 into a single mass. Unlike the Prawn, the Crab does not go 



* For an account of the various forms of the gastric apparatus in the 

 different families and genera of Brachyura, vide Ernot Nauck, " Das 

 Kaugerust der Brachyuren," Leitschrift f. Wiss. Zool., xxxiv., pp. 1-69. 



