340 THE INVERTEBRATA 



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. 233, apo.; 290, 

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

 the Notostraca, a mesodermal tendinous plate, the endosternite y 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. 309), to the ex- 

 tremest concentration. That of the Branchiopoda (Fig. 210) 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 crustaceans 

 various degrees of concentration of the ventral ladder are found, be- 

 ginning with the establishment of a suboesophageal ganglion for the 

 somites of the mouth parts (Fig. 225, s.oes.), and ending in the forma- 

 tion, in the crabs (Fig. 290) and some other forms, of a single ventral 

 ganglionic mass. In the Rhizocephala one ganglion (Fig. 261, ga.) 

 supplies the whole body. The brain contains ganglia for the eyes 

 (optic lobes), for the first or preantennulary somite (protocerebrum) ,^ 

 and for the antennules (deuto- or mesocerebrum). Except in the Bran- 

 chiopoda it also contains the antennal ganglia {trito- or metacere- 

 brum). A visceral ("sympathetic") system is present. In its main 

 features the functioning of the nervous system resembles that of 

 insects (p. 446). 



Sense organs are well developed in the free members of the group. 

 Eyes are of two kinds, the compound eyes, of which a pair is usually 

 present except in the Copepoda and adult cirripedes, and the median 

 eye. Details of the structure of the compound eyes have been given 

 above (p. 310). They may be sessile or stalked, and the latter con- 

 dition has given rise to a theory that they represent a pair of append- 

 ages. Since, however, there are no somites corresponding to their 

 ganglia and since at their first appearance in the embryo they are 

 sessile, this view is not generally accepted (see also p. 312). The 

 median eye (Fig. 226) is the eye of the Nauplius larva, and it persists 

 in most adults, though it is generally vestigial in the Malacostraca. It 

 consists of three pigmented cups, one median and two lateral, each 



^ As in other arthropods, the name procerebrum is given to the anterior 

 part of the brain, composed of the protocerebrum, the optic lobes, and some- 

 times other gangha which are not connected with paired hmbs. 



