380 INVERTEBRATE MORPHOLOGY. 



head-metameres unite with those of the two anterior thoracic 

 segments to form a single ganglionic mass lying behind the 

 oesophagus and sending nerves to the appendages of the 

 somites represented in the fusion. Similarly, in the posterior 

 region of the body of the Isopoda all the ganglia of the 

 abdominal region may fuse to a more or less simple mass 

 (Pig. 170, A, ah), and an extreme condition of fusion is to be 

 found in some Crabs (Fig. 170, B), in which all the ganglia 

 behind the antennary segment fuse to a single mass {tab), 

 lying in the thorax — a condition standing in relation to the 

 reduction of the abdomen and the extensive concentration of 

 the head and thoracic regions which are characteristic of 

 these forms. 



A sympathetic nervous system seems to be generally 

 present, consisting in its most complete condition of an un- 

 paired nerve arising from the syncerebrum and passing back- 

 wards to be distributed to the stomach, and of a median 

 nerve (Fig. 170, A, m) extending from one pair of postoesoph- 

 ageal ganglia to the other, lying between the two connectives. 

 Sense-organs reach a high degree of development in the 

 group. Hairs occur in abundance on the appendages and 

 body, the majority no doubt having merely a mechanical 

 function ; but among them will be found some beneath which 

 lie one or more ganglion-cells, giving rise to a nerve which 

 passes into the hair. These hairs are supposed to be tactile 

 in function. On the antennules of many forms and more 

 rarely upon the antennae, hairs of special forms occur, usually 

 in bunches or in rows. They may be club-shaped or cylin- 

 drical, and each has a nerve-fibre extending into it without 

 dilating into a ganglion-cell beneath its base. To these hairs 

 an olfactory function has been assigned, and it is noticeable 

 that they are usually more abundant upon the antennules of 

 the males than on those of the females — an arrangement 

 which suggests a probable service as guides in finding the 

 latter. 



Eyes are very generally present in the Crustacea, and 

 reach usually a high degree of efficiency. Two forms of eye 

 are known — a median unpaired one, frequently spoken of as 

 the simple eye, and the lateral or compound eyes. The un- 



