ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 829 



other it lias only one. Two commissures may also become connected, 

 and it frequently happens that they lie quite close to one another, so 

 that the distance between two commissures does not seem to be always 

 the same but to vary within wide limits. The pedal nerve-cords of 

 Turbo resemble those of Haliotis and have the same histological 

 disposition of a cortical layer of ganglionic cells, and an internal 

 nerve-plexus. 



The author defines the branchial ganglion as a nervous enlarge- 

 ment placed at the base of the anterior gill, which receives its com- 

 missure either from the sub- or supra-intestinal ganglion (Fissurella) 

 or, in the absence of these, directly from the similarly named com- 

 missures (^Raliotis, Trochidce) ; the presence of the subintestinal 

 ganglion appears to be a very primitive character, and when there is 

 a greater concentration of the nervous system, it becumes fused with 

 the supra-intestinal ganglion (Muricidee, Dolidse). These intestinal 

 ganglia were erroneously regarded by Simroth as pallial ganglia ; 

 where they seem to have disappeared the author imagines that they 

 are replaced by the scattered ganglionic cells found in the com- 

 missures. 



The views and descriptions given by v. Ihering of the pedal 

 nerves are discussed and criticized, and the conclusion is arrived that 

 at first these cords were not, in the oldest Gastropoda, placed on the 

 foot, but were situated deep in its musculature ; the difference be- 

 tween the Placophora and the Ehipidoglossa would appear to be due 

 to the atrophy of the inferior and the greater development of the 

 upper muscles of the body-wall, which became the strong shell-muscles. 

 Haller is of opinion that the transverse commissures between the 

 pedal cords of the Gastropoda, which are so well seen in Fissurella, 

 and erroneously described by v. Ihering in the Placophora, are not 

 primitive structures, which have been inherited from a Vermian 

 ancestor, but are more recent acquirements ; the plexiform arrange- 

 ment found in the Chiton must have preceded the more regular 

 commissural arrangement which obtains in Zeugobranchiate 

 Mollusca. 



After describing the details of the structure of the lateral organs 

 of this group the author proceeds to institute some comparisons 

 between them and the Vertebrata ; in both we find sensory organs 

 which are essentially formed of two kinds of cells — sensory and 

 supporting. The former are short clear structures (Mollusca) with a 

 .basal nucleus and long sensory hair, or their contents are strongly 

 granulated (Fishes). Basally, these cells lead to nerve-fibres, which 

 either arise from separate ganglionic cells, which lie beneath the 

 sensory organ (Mollusca), or from nerve-trunks without ganglionic 

 cells (Vertebrata). The supporting cells are long, clear, and cylin- 

 drical (Vertebrata, Fissurella), or their bodies are pigmented (Tro- 

 chidse) ; and they are never connected with a nerve-fibre. The number 

 of cells may be small (Vertebrata), or large (Mollusca). The absence 

 in molluscs of the protective organs which are developed in fishes 

 may be explained by the presence in the former of the superjacent 

 tentacles. 



