3S4 THE SENSORIAL FUNCTIONS. 



acquires still greater compactness, assumes a more globular 

 form, and has no central perforation. 



These different forms of structure are also exemplified in 

 the progress of the development of the liigher Crustacea: 

 thus, in the Lobster, the early condition of the nervous sys- 

 tem is that of two separate j^arallcl cords, each having a dis- 

 tinct chain of ganglia, as is the case in the Talitrus: then the 

 cords are observed gradually to approximate, and the gan- 

 glia on each side to coalesce, as represented in the Cymo- 

 thoa: and at the period when the limbs begin to be deve- 

 loped, the thoracic ganglia approach one another, unite in 

 clusters, and acquire a rapid enlargement, preparatory to the 

 growth of the extremities from that division of the body, 

 the abdominal ganglia remaining of the same size as before. 

 The cephalic ganglion, which was originall)^ double, and 

 has coalesced into one, is also greatly developed, in corre- 

 spondence with the growth of the organs of sense. The next 

 remarkable change is that taking place in the hinder por- 

 tions of the nervous cords, which are shortened, at the same 

 time that their ganglia are collected into larger masses, pre- 

 paratory to the growth of the tail and hinder feet; so that 

 throughout the whole extent of the system the number of 

 ganglia diminishes in the progress of development, while 

 their size is augmented. 



All Insects have the nervous system constructed on the 

 same general model as in the last mentioned classes; and it 

 assumes, as in the Crustacea, various degrees of concentra- 

 tion in the different stages of development. As an example 

 we may take the nervous system of the Sphmx ligusiri, of 

 which representations are given in the larva, pupa, and ima- 



feet (f.) and the abdominal nervous trunk (n;) the cephalic ganglion (c,) 

 communicating by means of two nervous cords (o,) which surround the 

 cEsophagus and entrance into the stomach (s,) with the thoracic ganglion 

 (b;) and sending ofT the optic nerve (e) to the eyes (e,) and the motor 

 ner\'es (m,) to the muscles of those organs; and also the nerves (a) to the 

 internal antennse, and the nerves (x) to the external antennsc (a.) 



