THE CRUSTACEA 



17 



arranged ganglia. In the primitive Branchio- 

 poda the ventral chain i-etains the ladder-like 

 arrangement found in some Annelids and lower 

 Avorms, the two halves being widely separated 

 and the pairs of ganglia connected together 

 across the middle line by double transverse 

 commissures (Fig. 11). In the other groups 

 the two halves of the chain are approximated 

 and more or less completely coalesced, and, in 

 addition, a concentration of the ganglia in a 

 longitudinal direction takes place, leading 

 idtimately, in many cases, to the formation of an 

 unsegmented ganglionic mass. This is seen, for 

 example, in the Brachyura, among the Decapoda. 



The brain consists, in the Branchiopoda, 

 mainly of two pairs of ganglionic centres, the 

 protocerebrum and deuterocerebrum (Fig. 2, 

 P, D), giving origin, respectively, to the optic 

 and antennular nerves. The antennal nerves 

 arise, in this group, from ganglionic swellings 

 on the oesophageal connectives. In the higher 

 groups, as already mentioned, the centres for 

 the antennal nerves have moved forwards and 

 are included in the brain, forming the trito- 

 cerebrum (Fig. 2, T), and other additional 

 centres are developed, so that in the highly 

 organised Decapoda the brain assumes an 

 extremely complicated structure. 



Etjei. — Two kinds of eyes are found in 

 Crustacea, the unpaired mexlitm or nau'plins eye, 

 and the paired compound eyes. The former 

 alone is present in the nauplius larva, and it 

 forms the sole organ of vision in the adult 

 Eucopepoda. It may coexist with the paired 

 eyes as in the Branchiopoda and in some of 

 the more primitive Malacostraca, although, in 

 the latter, it is generally vestigial. When 

 fully developed (Fig. 1 2), it usually presents 

 three divisions, each consisting of a cup- 

 shaped mass of dark pigment {])), the cavity 

 of which is filled wdth columnar retinal 

 cells. The outer ends of these cells are con- 

 tinuous with the nerve-fibres (»), Avhile at 

 their inner ends they contain rod-like bodies ^y !''«'• «■"■*,, '» t•'i^ ■I'-^vr"^; ''/'" 



•^ .... „ , trf'iiiPlV'ioiibttul. rossibly the .stnictiiic 



(r). In some cases the three divisions or the observed may w a portion or 



Nervous system of Jiraiifliiiircia 

 p(diiilo!<<i, one of tlie Branchiopoda (after 

 .Sars), sliowinj; the laihler-like form .■of 

 the antei'ior jiart of the ventral neive- 

 c'hain and the absence of gan^jlia and 

 of transverse connnissnres in tlie pos- 

 terior limbless part of the trunk, hi 

 indicates the position of the mouth. 

 The existence of a transverse com- 

 missure in front of the mouth, as shown 



bserved maj' be 

 visceral nerve-rini; encircling 



h, . T 1 , visceral nerve-rini; encirclin'r 



supplied by a separate nerve in the region of ti.e labrmn. 



the 

 the KuUet 



