600 CLASS X. 



antennae are short, in others very long ; the external are commonly 

 the longest, and in the genus Palinurus exceed twice the length of 

 the body. In some genera there are only two antenna? present; 

 the genus Limulus has no antenna at all. 



There are commonly two compound eyes, or two groups of 

 simple eyes. The simultaneous presence of two compound eyes 

 with simple eyes, so common in the winged insects, is observed in 

 only very few crustaceans. In most of these there are only two 

 compound eyes, which are often fixed upon a pedicle and moveable. 



The mouth of crustaceans is situated on the inferior surface of 

 the anterior portion of the body. The oral organs or jaws are 

 arranged in pairs and move sideways, as in manducating insects. In 

 some crustaceans, however, these parts acquire a modified form, and 

 compose a sucker for taking up fluids, on which these animals live. 

 The description of the jaws may well detain us some moments, and 

 requires a more particular survey according to the different orders. 



In the ten-footed crustaceans (lobsters and crabs) a transverse 

 upper lip is present ; beneath it lies a pair of upper jaws, which are 

 very hard, cut off straight at the extremity, and at the upper margin 

 provided with a short feeler of two or three joints. [Since the 

 upper jaws (mandibulce) in insects are without feelers, it has been 

 thought that these joints do not represent & palpus, but a continua- 

 tion of the pedicle of the jaws : at all events this palpus differs from 

 that which occurs on the accessory under-jaws of crustaceans.] 



A thin, membranous, bifid tongue, is situated at the base of the 

 upper jaws 1 . To it there succeed a first pair of under-jaws, which 

 are membranous, divided into lobes and beset with hairs at the 

 margin. Next follows a second pair of under-jaws, which are also 

 membranous and haired, and may be compared with the under-lip 

 of insects which has been split into two parts. Then follow three 

 pairs of jaws, or rather of feet changed into jaws, to be compared 

 with the six feet of insects. At the outside of these accessory jaws 

 a palpus is attached, which, as external division of the foot, does 

 not correspond with the palpi of insects, and was named flagrum by 

 SAVIGNY. This flagrum consists of a flat, elongated part at the 

 basis, and a many-jointed filament running to a point at the end. 



1 COVIER calls this part sixi&me mdchoir, Lef. d'Anat. comp. in. p. 304 ; FABBI- 

 CIUS gave it the name of lohiinn. 



