704 



CRUSTACEA. 



But it IB more especially in the types which 

 still ask our attention, that we perceive the 

 system of centralization pushed yet farther by the 

 actual conjunction of the nuclei, which we have 

 hitherto only seen approximated to one another, 



1 O 



*.._-; 



ft* 



7 



Nervous system of the Astacus Marinus, or Sphinx 

 Ligustri. 



in consequence of their gliding or encroaching, 

 as it were, upon the median line. 



The Lobster (Astac us marinus) (fig. 39 3) pre- 

 sents us with another step in the system of cen- 

 tralisation. Here, in fact, the longitudinal cords 

 of communication are entirely consolidated 

 along the median line through the whole of the 

 abdomen, although they are still to be found 

 double in the thorax. Moreover, the first thoracic 

 ganglion ( 4 ), and the last of the abdominal 

 series of ganglions (a c ), are conspicuously 

 formed by the reunion of several distinct ner- 

 vous centres, in the way we have already indi- 

 cated as happening, although in a minor degree 

 and less perfectly, in the Amphipoda and the 

 Isopoda. Before we pass, however, to the con- 

 sideration of more complicated systems, we shall 

 pause a moment to describe somewhat at length 

 the one which we have but just mentioned, 

 the more as it is among the number of those 

 which have been most attentively studied. 



The cephalic ganglion (g ! ,/g. 393), situated 

 above the base of the internal antennae, is of con- 

 siderable size, and appears to be simple ; it gives 

 origin to five pairs of nerves and to two cords, 

 which connect it with the rest of the ganglionic 

 nervous system. The first of these pairs (o)arises 

 from its anterior edge : this is the optic pair, 

 which, after having penetrated the peduncles of 

 the eyes, increase in size, and traverse a mem- 

 branous diaphragm, which may be likened to 

 the sclerotic coat. 



The second pair of nerves correspond to the 

 ocular motors ; they run parallel to the pre- 

 ceding pair, and are distributed to the muscles 

 of the eyeball. 



The third pair proceed to the internal anten- 

 nae (6) ; but before they enter these appendages 

 they send off a branch to the muscles which 

 move them. A like ramification is sent off 

 from the principal trunk to each of the rings of 

 which these antennae are composed, and the 

 nerve ends by becoming bifurcated, in order to 

 penetrate the two filaments in which the an- 

 tennae terminate. 



The fourth pair of nerves (e) are distributed to 

 the tegumentary membranes of the anterior 

 extremity of the animal. Behind the fourth a 

 fifth pair is seen (d), which proceeds anteriorly to 

 the fourth pair almost immediately after its 

 origin, sends one branch to the cake-like organ 

 of doubtful function which covers the ear, a 

 second branch to the organ of hearing itself, 

 and finally terminates in a trunk of considerable 

 size, which traverses the external or second 

 antenna through its entire length. 



A sixth pair is destined to establish con- 

 nexions between the cephalic ganglion and the 

 first of the thoracic ganglions, after having sur- 

 rounded the oesophagus ; but instead of ap- 

 pearing as simple nervous cords through their 

 whole length, as in types which we have 

 hitherto studied, each of them presents an 

 enlargement in its middle, which is neither 

 more nor less than a ganglion, from which there 

 is sent off, first, a nerve that proceeds to the 

 mandibles ffj; next, a gastric nerve (g), of large 

 size, which gives many filaments to the coats 

 of the stomach, and finally anastomoses with 



