CRUSTACEA 303 



mouth but may serve other ends. The maxillae have various functions 

 in connection v^^ith feeding and respiration. The limbs of the thorax 

 perform in various cases practically every function for v^hich ap- 

 pendages are used. If a crustacean walks, it is usually by means of 

 these limbs. Often in one or more of them the last joint can be 

 opposed to the joint which precedes it, forming a chela (or a subchela)^ 

 so that the appendage is adapted for grasping. Modification of the 

 hinder thoracic or anterior abdominal limbs in connection with repro- 

 duction is common. Abdominal limbs are lacking save in certain of 

 the Branchiopoda and most of the Malacostraca. When they are 

 present they are commonly used for swimming, for setting up currents 

 of water, or for carrying eggs and young. 



Three elements of minor importance complete the external make-up 

 of the Crustacea. In front of the mouth is a labrum or upper lip; 

 behind the mandibles is a lower lip or metastoma^ usually cleft into 

 a pair of lobes known as paragnatha ; and on the telson usually (but 

 in no adult malacostracan except the Leptostraca Fig. 254), is a pair 

 of caudal rami forming the caudal furca. 



Appendages which are lost are regenerated at subsequent moults ; 

 and the highest members of the group possess an elaborate mechan- 

 ism for autotomy — the breaking-off of limbs which have been injured 

 or which have been seized by enemies. 



An internal skeleton is usually present in the form of ingrowths of 

 the cuticle, known as apodemes, which serve for the insertion of 

 muscles. Sometimes (notably in the Decapoda, Figs. 222, apo.; 276, 

 enph.) they unite to form a framework, the endophragmal skeleton. In 

 the Notostraca, a mesodermal tendinous plate, the endosternite , lies 

 under the anterior part of the alimentary canal. 



The nervous systems of Crustacea exhibit a very complete series of 

 stages from the ideal arthropod condition (see p. 274), to the ex- 

 tremest concentration. That of the Branchiopoda (Fig. 199) is in a 

 very primitive state, having the antennal ganglia behind the mouth as 

 the first pair of the ventral ladder, distinct ganglia for the following 

 somites, and widely separated ventral cords. In the lower members 

 of the Malacostraca (Nebalia, some mysids, etc.), the antennal 

 ganglia have joined the brain and the ventral cords are closer together, 

 but otherwise the primitive condition is retained. In other crus- 

 taceans various degrees of concentration of the ventral ladder are 

 found, beginning with the establishment of a suboesophageal 

 ganglion for the somites of the mouth parts (Fig. 214,5.0^5.), and ending 

 in the formation, in the crabs (Fig, 276) and some other forms, of 

 a single ventral ganglionic mass. In the Rhizocephala one ganglion 

 (Fig. 250, ga.) supplies the whole body. The brain contains ganglia 

 for the eyes (optic lobes), for the first or preantennulary somite (pro- 



