MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



293 



j)roved by Pelseneer to contain the antenna! ganglia, though the nerves branch off at a consid- 

 erable distance from the brain, on the side of the a?sophagus. A step higher is seen in the brain 

 of isopods, where, as we have described and figured * the first and second antennal ganglia of 

 Asellus are not closely coalesced and consolidated with the optic and cerebral lobes, but are sepa- 

 rate from the biain and situated below and posterior to it, though in front of the oisophagus. The 

 most complex tyjie of crustacean brain is that of the highly specialized Decapoda. 



The brain of adult insects is on the same high plane as that of decapod Crustacea, the ojjtic 

 and antennal lobes being invariably coalesced with the cerebral, the whole mass being separated 

 from the infra-cesophageal ganglion, and so it is with the brain of myinopods. 



The Arachnidan brain is very distinct in its external appearance from that of insects or 

 Crustacea. 



Prof. Pattent has given us what is apparently a clear and accurate figure on an enlarged 

 scale of the scorpion's central nervous system, which we take the liberty to copy, with a 

 change in position and consequent change in the lettering. 



in-.st, 



Fig. IV — Brain of adult scorpion, constructed by means of sections and dissections. A. B., accessory brain ; H. B.,hind brain; M. B., mid 

 brain; F. B., fore brain ; a. co., anterior commissures; a. A. ?i. 1-6, anterior breraal nerves of tlie thoracic ueuromeres; c. sy. cranial sympa- 

 thetic (?) ; p^, large ganglion derived from the segmental and coxal sense organs, and giving off many branches to the scattered sense organs 

 in skin of ohelfe; (?./., ganglion fusiforme; g. nod., ganglion nodosum or g. striatum; A. ?i. «., a very delicate root, probably representing the 

 h;£mal nerve of the third fore-brain neuromcre, partially fused with the h^mal nerve of the cheliceral segment; h. v.' and U. v.", the inde- 

 pendent anterior and posterior nerves of the first vagus neuromere ; h. vS" and h. y.'", the four partially fused hiemal nei-ves of the second and 

 third vagus neuro-oieres ; lat. e. n., lateral-eye nerve; ?», mouth; me. n., median-eye nerve; m. st. n., median stomodieal rostral nerve; n. e., 

 comb or pectinal nerve; ?i(7.i-6, neural ganglia at the base of the neural nerves; n. n.i-tj, neural nerves of the thoracic segments; k. t).i-4, 

 roots of the neural vagus nerves; ph. n.i-f, posterior hicmal nerves of thethorax; p. co., posterior brain commissure; «. I. (str. body), semi- 

 lunar lobe or first brain segment; r. m., retractor muscles to cesophagua. — After Patten. 



Prof. Patten incorrectly applies the term "brain" to the entire central nervous system, only 

 excepting the abdominal ganglia, dividing the brain and oesophageal ring or mass into three divis- 

 ions, viz, the fore, mid, and hind brain, as indicated in his figure. But restricting the term brain to 

 the supra-cesopbageal ganglionic mass, it will be seen that the ganglia which innervate the first 

 pair of appendages, though separate in the embryo, are in the adult scorpion closely united with 

 the brain, though there is a slight constriction between the two portions of the brain, viz, between 

 the "mid brain" (or rostro-mandibular ganglion of St. Remy) and the "fore brain." 



It is desirable also, for the sake of cleai-ness, not to consider the "hind brain" as a part of 

 the brain, since it is a post-oesophageal mass, innervating the cephalic and thoracic appendages, 



* structure of the braiu of sessile-eyed Crustacea, 

 t Quart. Jouru. Micr. Sc, 317, 1890. 



