Dr. J. G. Jeffreys on a new Species of Chiton. 33 



The asymmetry of the brain is remarkable : the large 

 ganglionic cells are most abundant in the centre behind the 

 middle, and from there to the posterior side of the brain a 

 median line is slightly indicated by the arrangement of the 

 nucleogenous bodies. The tract composed of large nerve- fibres 

 with scattered ganglionic cells on the left side is very much 

 more extensive than on the right. 



Comparison with the Brain of other Arthropods. — So wholly 

 unlike, in its form, in the want of antennal nerves, and in 

 internal structure, is the supracesophageal ganglion or " brain " 

 of Limulus to that of insects and the higher Crustacea, that it 

 is very difficult to find any points of comparison. 



Histologically, judging by my specimens of the brain of 

 the lobster which are stained with carmine, the brain of 

 Limulus agrees with that of other Arthropods in having 

 similar large ganglion-cells ; the smaller ganglion-cells, so 

 abundant in the brains of insects and Crustacea, are wanting 

 in Limulus. There are in Limulus no " ballen-substanz " 

 masses homologous with those of the other Arthropods. 



Topographically the internal structure of the brain of 

 Limulus is arranged on a wholly different type from that 

 of any other Arthropodous type known — so much so that it 

 seems useless to attempt to homologize the different regions 

 in the two types of brain. The plan is simple in Limulus, 

 much more complex in Arthropods, especially in the brain of 

 the crawfish as worked out by Krieger, as in the Decapodous 

 brain there arise two pairs of antennal nerves besides the 

 optic pair ; and in external form the two types of brain are 

 entirely unlike. The symmetry of the brain of the crawfish, 

 as of the lobster and insects, is marked throughout, each 

 hemisphere exactly repeating in its internal topography the 

 structure of the opposite side ; that of Limulus is obscure and 

 imperfect. 



VII. — On a new Species of Chiton lately found on the 

 British Coasts. By J. Gwyn Jeffreys, LL.D., F.R.S. 



Chiton scabridus*. 



Body thin, semitransparent, of a blood-red colour : mantle 

 dirty white : mouth small : foot lanceolate, only one third the 

 width of the body, tapering rather gradually to a fine point; 

 the sole is marked lengthwise by six red lines, which are 



* Rough. 

 Ann. & Mag. N. Hist. Ser. 5. Vol. vi. 3 



