12 



NERVOUS SYSTEM AND SENSE-ORGANS 



Op.n 



advantageous in regard to correlation of the different parts of the 

 body, and adaptation to the environment. It may also be noted 

 that while in many lower Arthropods the two halves of the ventral 

 cord are more or less separate, they are intimately united together 

 in higher forms. 



Nervous Systems of Crustaceans (Crustacea). Successive 

 stages of fusion in the nervous system may be illustrated by 

 comparison of Apus, Crayfish, and Crab. In the first of these, 



which is one of the lower forms, there is a 

 nerve -ring with clearly -marked brain, and 

 a ventral cord of which the two halves are 

 widely separate (fig. 1014). The brain of a 

 typical Annelid, such as the Sea-Centipede 

 (Nereis], is lodged in a head-lobe (prosto- 

 mium) that forms the front of the head and 

 overhangs the mouth, and it supplies with 

 nerves the eyes and feelers which are borne 

 upon this lobe (fig. 1014). The brain of 

 Apus is placed in a corresponding position, 

 and is in the main equivalent to that of 

 Nereis, though probably not entirely so. A 

 Crustacean possesses two pairs of feelers 

 (antennules and antennae) situated in front 

 f the mouth, but. most likely their original 



Gn.i 



Gn.2 



'"Gn.4 



Fig. 1014. Front Part of the 

 Central Nervous System of Apus, 

 enlarged 



G., piace where guiiet runs 

 rough nerve-ring; Gn.i-Gn. 4 , 



ganglia of one half of ventral cord; 



through nerve-ring; Gn.i-Gn. 4 , position was behind that aperture, and they 



J 



op.n, Ant.r, Ant. 2 , Md., MX.I, have shifted forwards into a position more 



MX. 2, Th.f.i, nerves to eyes, an ten- t_l / 1 11 



nules, antenna, mandibles, first and Suitable IOr tHC WOrK they 



1 

 the 



second maxillae, and first thoracic 

 feet; V., visceral nervous system. 



r 



tO perform, 

 * r 1 



by Way Ot CXplOHng 



These two pairs of feelers belong to two 



segments of the head, each of which is provided with a cor- 

 responding pair of ganglia. In Apus those of the segment to 

 which the antennae belong are the first pair of the ventral cord 

 (see figure), but the nerve for each front feeler or antennule arises 

 from the side of the nerve-ring, and can be traced into the brain. 

 This is intelligible if we suppose that organ to be equivalent to the 

 brain of an Annelid, plus the ganglia supplying the antennules, 

 which have shifted forwards along the sides of the nerve-ring. If 

 this view be correct, a certain amount of fusion and centralization 

 has taken place at the front end of the nervous system in Apus, as 

 compared with an Annelid. But it is here necessary to state that 



