28 COMPARATIVE ANATOMY. 



but one. The nervous system is seen at intervals underlying 

 the digestive tract in the abdomen. 



47. A gravid female Scorpion, dissected to shew the posi- 

 tion of the embryos in an ovi-viparous Arachnid. 



The dorsal surface of the animal has been removed. The 

 embryos, differing from the parent only in size, are seen occupy- 

 ing the ovarian tubes, which overlie the liver or are even em- 

 bedded in its substance. 



48. A Spider, dissected to shew its nervous system. 



The whole of the post-oral ganglia are seen to be con- 

 centrated into a single stellate mass, occupying the larger 

 portion of the ventral surface of the cephalo-thorax. From 

 this mass, which may be taken to represent the confluent 

 thoracic and abdominal ganglia of Insecta, radiate, on each 

 side, five principal nerves to supply the legs and maxillary 

 palpi. Several nerves pass posteriorly from its hinder edge to 

 supply the abdomen. The cephalic ganglia (two in number) 

 are situated on the anterior edge of this stellate mass, between 

 which and the former the oesophagus passes. From this pair 

 of ganglia nerves are seen passing forwards to supply the eyes 

 and mandibles. 



Division ARTHROPODA BRANCHIATA. 



Class CRUSTACEA. 

 1. Order Decapoda. 



49. A Lobster (Homarus vulgaris), disarticulated, so as 

 to shew its segmentation. 



The different segments with their appendages have been as 

 far as posible separated from each other, and serially arranged 

 in their natural order. The segments have been numbered 

 from the first, or ophthalmic segment, backwards. The carapace 

 has been removed, and fixed on the right of the sub-abdominal 

 segments. 



The skeleton consists of a broad, shield-like carapace, 

 1 cephalo-thorax/ and of the six segments, or 'somites/ which enter 

 into the composition of the post-abdomen. Attached to the 

 ventral surface of these different segments are the variously 



