544 THE SENSORIAL FUNCTIONS. 



PhyUosoma (Leach), where the gangha compos- 

 ing each pah* in the abdomen and in the head, 

 are united into single masses, while those in the 

 thoracic region are still double. In the Cymo- 

 thoa (Fab.), which belongs to the family of 

 Oniscus, there is the appearance of a single chain 

 of ganglia, those on the one side having coa- 

 lesced with those on the other; each pair com- 

 posing a single ganglion, situated in the middle 

 line ; while the longitudinal cords which connect 

 them still remain double, as is shown in Fig. 440, 

 which represents the interior of this crustaceous 

 animal, nearly of the natural size. But in the 

 higher orders of Crustacea, as in the Lobster, 

 these longitudinal cords are themselves united in 

 the abdominal region, though still distinct in the 

 thorax. 



In following the ascending series of crustace- 

 ous animals, we observe also an approximation 

 of the remoter ganglia towards those near the 

 centre of the body : this tendency already shows 

 itself in the shortening of the hinder part of the 

 nervous system of the Cymot/ioa, as compared 

 with the Talitrus; and the concentration proceeds 

 farther in other tribes. In the Palemo/i, for 

 example, most of the thoracic ganglia, and in the 

 Palinunis (Fab.), all of them, have coalesced 

 into one large oval mass, perforated in the 

 middle, and occupying the centre of the thorax ; 



