170 LECTURE XIV. 



are developed on the abdominal chords, which are continued along 

 the muscular tail : the last, which is above the anus, is the largest, 

 and radiates the nerves to the terminal swimming plates of the tail. 

 This is probably a coalescence of the originally distinct ganglions of 

 the sixth and seventh segments of the abdomen or tail. 



The nervous system of the lobster is displayed in these dissections 

 by John Hunter* and Mr. Swan, in whose work on Comparative 

 Neurology it is beautifully illustrated. The nerves of the lobster 

 closely correspond with those of the fresh-water species, or craw-fish. 

 The non-ganglionic tracts are shown in this dissection of the lobster 

 by Mr. Newport f ; but the distinctions of the origins of the nerves 

 from the dorsal and the ventral tracts are, as Mr. Swan remarks, by 

 no means clear. The oesophageal columns are united in both species 

 of Astacus by a transverse commissural chord. 



In the prawn {Palemon) and rock-lobster (Palinurus), the thoracic 

 ganglia coalesce to form a long, elliptical, perforated nervous mass. 

 In this dissection of a hermit-crab (^Pagurus^X, the cephalic ganglion 

 presents a transversely quadrate form^ and sends off the usual nerves 

 to the eyes, the ears, and the antennae. The lateral oesophageal 

 chords, after supplying the digestive system with the stomato-gastric 

 nerves, unite below to form the ganglion which distributes nerves to 

 the maxillary apparatus and pharynx. This is succeeded by a large 

 oblong ganglion, situated at the base of the great nippers, and of the 

 second pair of feet, both of which pairs it sup^jlies. The lateral 

 chords diverge for the passage of the artery, re-unite to form a third 

 thoracic ganglion, smaller than the second, supplying the third pair 

 of thoracic legs, and sending off three pairs of nerves posteriorly. Of 

 these the latei'al pair goes to the fourth diminutive pair of feet ; the me- 

 dian pair supplies the fifth feet : the two remaining dorsal nerves, which 

 are of minute size, form the continuations of the abdominal chords, 

 and pass along the under or concave side of the soft, membranous, and 

 highly sensitive abdomen to the anus, anterior to which the last small 

 ganglion is situated : this supplies the nerves to the muscles of the 

 caudal plates, here converted into claspers, and enabling the animal 

 to adhere to the columella of the univalve shell, which it may have 

 selected to protect that portion of its body which nature has left 

 undefended by the usual dense and insensible crustaceous covering. 



Experiments have been made, and repeated, in order to determine 

 whether the ganglionic portions of the abdominal chords of the 

 Articulata have the same restricted function which the posterior 



* Preps. Nos. 1301, 1302, 1303. f Prep. No. 1302, A. 



\ Prep. No. 1303. B. 



