ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 373 



Grenacher's embryological investigations to have no certain basis, yet he 

 perceives that there is at least a possibility of characterizing a propodium 

 by anatomical characters. 



The physiological significance of the nervous plexus is very difficult 

 to explain. Where it is highly developed, as in Harpa, Natica, &c., the 

 portion of the foot which contains it is provided with a wealth of nerves 

 such as there is not the like of among the Mollusca. The tendency of 

 nerves, especially in their terminal branches, to break up into a plexus 

 is very great in the Mollusca ; they have been described in the gastric 

 and enteric walls of various Proso- and Opisthobranchiata, and Pulmo- 

 nata, in the branchiae, pericardium, wall of the heart, mantle, lips and 

 kidney of Prosobranchs, and elsewhere. 



An account is given of the central nervous system and the visceral 

 commissure of Pteroceras, which present some abnormal characters. 

 Instead of a ganglionic mass above and below the enteric canal, there 

 is one to the right and another to the left of it. The commissure which 

 generally goes from the right pleural ganglion to the supraintestinal 

 ganglion appears to be wanting, and instead of it a strong nerve passes 

 from the left pleural to the subintestinal ganglion. The difficulties 

 raised by these peculiar arrangements are explained when an examina- 

 tion is made of the ganglia of the oesophageal ring, vphich has been 

 twisted through a right angle in the direction opposite to that of the 

 hands of a watch. A similar alteration has been observed in Strombus 

 luhuanus. 



We now know of three ways in which the simple, typical, visceral 

 commissures of Prosobranchs may be made more complicated. The 

 first consists in a fusion of the parts of the hinder loop, together with 

 a marked shortening of the anterior. This very peculiar differentiation 

 leads to the apparently orthoneural visceral commissure of the Neritina 

 and Helicina. If Pelseneer's views are correct, the Heteropoda would 

 form a kind of intermediate stage. The second mode of differentiation 

 is that here described ; it ends in the anterior loop of the visceral com- 

 missure, being placed to the left of the intestine, and this is seen in the 

 CypragEB and Alata. A third method is the approximation and fusion of 

 the supra- and subintestinal ganglia with the pleural ganglia of their 

 proper side ; this may be seen in all stages in many of the higher Proso- 

 branchiata. It generally happens that the subintestinal ganglion first 

 fuses with the right pleural ganglion, while the supraintestinal long 

 remains independent. The final result is the extreme shortening of the 

 anterior loop of the visceral commissure. 



Hermaphroditism of Aplysiae.* — M. E.Eobert has some observations 

 on the recent paper of M. Kemy Saint-Loup.j He does not for a moment 

 doubt that Aplysia fasciata or A. depilans have the sexes united, and the 

 information which can be got by examining the hermaphrodite gland 

 may be supplemented by a study of the conformation of the accessory 

 reproductive organs, which are altogether on the hermaphrodite type. 

 No difference can be observed between the individuals in copulation. 



M. Saint-Loup's error appears to have arisen from his taking for 

 males younger animals than those which he calls females ; the latter are 

 adults perfectly hermaphrodite. In young forms spermatoblasts and 



* Comptes Eendus, cviii. (1889) pp. 198-201. 

 t See this Journal, ante, p. 195. 



1889. 2 D 



