MOLLUSCA. 



275 



With reference to the first of these points it should be noticed that the 

 supra-cesophageal and pedal ganglia are at first respectively connected with 

 the optic and auditory organs, and that these sense organs are in some cases 

 at any rate developed anteriorly in point of tinie to the ganglia. It seems 

 perhaps not impossible that primitively the ganglia may have been simply 

 differentiations of the walls of the sense organ, and perhaps their apparent 

 derivation from the mesoblast is really a derivation from cells which 

 primitively belonged to the walls of these sense organs. Bobretzky's 

 observations on Fusus fit in well with this view. 



In the Hyaleacea and in other Pteropods, where the eyes are absent in 

 the adult, Fol finds the supra-oesophageal ganglia resulting from a pair of 

 epiblastic invaginations. May not these invaginations be really rudiments 

 of the eyes as well as of the ganglia ? Fol also, it is true, describes a similar 

 mode of origin for these ganglia in Limax. It would be interesting to have 

 further observations on this subject. The independent origin of the pedal 

 and supra-cesophageal ganglia finds its parallel amongst the Chsetopoda. 



Int* 



Co. ep C 



FIG. 122. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA. 



(After Grenacher.) 



A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod. 

 Pal. eyelid ; Co. cornea; Co.ep. epithelium of ciliary body ; Ir. iris; /;//. 7/// 1 ... 

 7;;/ 4 . different parts of the integument ; /. lens ; 7 1 . outer segment of lens ; R. retina ; 

 N.op. optic nerve; G.op. optic ganglion; x. inner layer of retina; N.S. nervous 

 stratum of retina. 



The supra-cesophageal ganglia appear always to develop within the 

 region of the velar area. This area corresponds with the prae-oral lobe of 

 the Chaetopod larva, at the apex of which is developed the supra-cesophageal 

 ganglion. Embryology thus confirms the results of Comparative Anatomy 

 in reference to the homology of these ganglia in the two groups. 



Optic organs 1 . An eye is present in most Gasteropods and 



1 For a fuller account of this subject the reader is referred to the chapter on ' The 

 Development of the Eye.' 



I 8 2 



