584 SUMMARY OF CURRENT RESEARCHES RELATING TO 



are placed the ganglia, and the two triangles are united across the middle 

 line by two commissures. The visceral commissure is long, and, while 

 connecting the pleural ganglia, has a number of ganglia (visceral) 

 developed on it. In the forms under examination (Chiastoneura) the 

 arm of the commissure from the left pleural ganglion passes ventrally 

 and to the right, and then bends towards the dorsal surface, goes to 

 the left, and extends to the right pleural ganglion. On this two or 

 three ganglia are developed. From one, the supra-intestinal, a strong 

 nerve gives off branches to the gills and left half of the mouth, but 

 goes chiefly to the so-called " rudimentary gill " or colour-gland of 

 T. Williams. This is the olfactory organ of Dr. Spengel, who finds 

 that it consists of a large mass of nervous substance, invested by a 

 layer of epithelium, into which nerve-fibres distinctly pass. 



In examining the Orthoneura, the writer finds that this organ is 

 more complicated in structure; for example, there is an enormous 

 ganglion in the centre, with a central fibrous mass and peripheral 

 cells, around which are pinnate filaments of connective tissue, lacunae, 

 and ciliated epithelial cells ; nerve-fibres pass into the lamellae of the 

 filaments. The author then proceeds, by an account of the arrange- 

 ment of the nervous system in the Orthoneura, to demonstrate the 

 homology between this organ and the similar one in the Chiastoneura, 

 and finds in addition that the symmetry of the visceral nervous system 

 is due to secondary connections between its ganglia and the pleural 

 ganglia ; probably there are no true Chiastoneura, in the sense in 

 which that word is used by v. Ihering. 



We arrive, therefore, at this first general conclusion — that the 

 classification of certain groups of the Gastropoda into Chiastoneura 

 and Orthoneura is untenable. 



The author then discusses the Heteropoda, where he finds the 

 nervous system to consist of two cerebral and two pleuropedal ganglia, 

 between which the typical connective and commissural fibres may be 

 distinctly developed, or almost lost ; but in all cases, a nerve for the 

 olfactory organ is given off from the supra-intestinal ganglion ; the 

 arrangement in the Heteropoda being referable to what obtains in the 

 Prosobranchiata, it is concluded that they are Prosobranchs modified by 

 adaptation to the pelagic mode of life. The only exception is to be 

 made for the Zeugobranchiata {Haliotis and Fissurella), which have 

 two gills developed, one on either side, with which may be associated 

 Patella ; to understand these, however, it is necessary to make refer- 

 ence to the torsion of the body which appears to have taken place. 

 This appeal's to have caused the gills and olfactory organs to move 

 round a circle the centre of which is the anus, and the effect of this is 

 to bring the left gill and the connected olfactory organ in front of the 

 anus to the right side, and the right gill and its olfactory organ to the 

 left. The Anisobranchiata are Zeugobranchs in which the organs of 

 the right side have disappeared, and those of the left have taken on a 

 compensatory greater development. 



After discussing other groups, the author comes to the considera- 

 tion of the Pulmonata ; here he again finds that the " nouvel organe 

 d'innervation " of Lacaze-Duthiers is olfactory ; owing to the want of 



