310 LECTURE XIV. 



appendage, on entering which, it is resolved into a plexus or kind of 

 " Cauda equina." Besides the principal nerves above mentioned, many 

 smaller nerves are given off to other parts of the body. The sides 

 of the great oesophageal ring are united by two transverse com- 

 missural bands : but the most remarkable feature of the nervous 

 axis of this Crustacean is its envelopment by an arterial trunk. A 

 pair of aortae from the fore part of the heart arch over each side of 

 the stomach, and seem to terminate by intimately blending with 

 the sides of the cesophageal nervous I'ing. They, in fact, expand upon, 

 and seem to form its neurilemma ; a fine injection thrown into them 

 coats the whole central mass of the nervous system with its red coloui\ 

 Such is the condition of this governing part of the organisation in 

 the most gigantic form of the Entomostracous tribe, and probably the 

 only existing genus from which we may derive an insight into the 

 structure of the extinct Trilobitic Crustaceans. 



Three principal divisions of the nervous system of the Crustacea 

 may be defined according to their probable functions. Thus, ad- 

 mitting, from analogy, that the super-ojsophageal ganglionic centre 

 {figs. 129. and 131. c) is that in which true sensation and volition 

 reside, then those nervous filaments which are exclusively connected 

 therewith, and some of which would seem to extend the whole length 

 of the animal along the dorsal aspect of the ganglionic columns, will 

 form with their ganglionic centre the true sensori-volitional system ; 

 wliilst any other ganglions superadded to the abdominal columns, 

 with the nervous filaments terminating in or originating from them, 

 will constitute the system for the automatic i-eception and reflection 

 of stimuli. The stomato-gastric nei-ves, connected partly with the 

 brain and partly with the cesophageal columns, will form a third 

 system analogous to the great sympathetic or organic nerves of the 

 Vertebrata. In these views I coincide with the accomplished phy- 

 siologist. Dr. Carpenter, believing that their accuracy and soundness 

 have received confirmation from the facts of Comparative Anatomy, 

 described in the previous pages, and which, in the Ilunterian 

 Lectures of 1842, were for the first time brought to bear upon this 

 interesting problem. 



The sense of touch can be but very feebly exercised by the com- 

 mon integument of the Crustacea, can hardly, indeed, exist except in 

 those parts of the surface of the body which remain soft and un- 

 defended by the hard crust, such as the joints of the under part of 

 the body, and the surface of the soft tail in the hermit crabs. The 

 fine hairs which project from many parts of the integument may 

 compensate for its low endowment of the tactile sense : the tAvo pairs 

 of jointed antennae {,figs. 128. 131. a) are instruments fitted for the 



