20 SUMMARY OF CURRENT RESEARCHES RELATING TO 



eye is completely formed ; it is placed in the concavity of the last 

 intestinal loop, and its upper extremity, which carries the pupil, is 

 placed at the level of the anus. The base is less strongly pigmented 

 than the rest, and has on its inner surface a small mass of cells, which 

 are found in section to bo insensibly continuous with the ectodermal 

 integument, and which must be considered as the rudiment of the 

 asymmetrical nerve-centre. Longitudinal sections of the organ show 

 that the upper half of the pigmented sac is entirely occupied by the 

 crystalline portion, while its inferior half is lined by a relatively thick 

 layer, which is finely dotted, and evidently represents a retina. 



It is clear that this organ presents all the essential parts of a highly 

 specialized eye, and there is no doubt that its duty is to make up for 

 the absence of the cephalic eyes, which are always wanting in the long 

 free larval life which is led by Philine. 



In Bulla hydatis there are two well-developed cephalic eyes, but, 

 nevertheless, the anal eye has the same structure and relations as in 

 Philine ; but it is interesting to remark that it has no function to per- 

 form, for the larva does not become free till the twenty-fifth day, and 

 the eye commences to atrophy before the embryo leaves the egg. 



As to the morphological significance of this organ, we are reminded 

 that Prof. Lacaze-Duthiers long since described, at the entrance of the 

 mantle-cavity of aquatic Pulmonates, a " special organ " in the form of a 

 vibratile pit set in a small ganglion ; this has always been since regarded 

 as having an olfactory function. As it is always proportionately larger 

 during embryonic life it has been regarded as a larval organ. With this 

 M. Pol has compared the ciliated pads, which have the same innervation 

 and appear to have the same function in Pteropods and Heteropods. It 

 seems to the authors that the anal eye of Opisthobranchs is in them the 

 representative of this structure, the physiological differences in no way 

 implying differences in morphological value. 



The otocysts of Philine are formed in exactly the same way as the 

 eye, the otolith appearing before the neighbouring cells surround it to 

 form the wall of the auditory vesicle, which only later becomes sunk 

 into the substance of the foot ; the pedal ganglion, as is the rule for 

 sense-organs of Gastropods, appears last. 



Nervous System of Aplysia.* — Prof. H. de Lacaze-Duthiers con- 

 tinues his morphological study of molluscs, and describes the anatomical 

 nervous relations found in Aplysia. 



The oesophageal commissure, at the level of the large tentacles and eyes, 

 has this first peculiarity, that the commissure of the pedal ganglia being 

 very long, these two centres become lateral. The two first ganglia of the 

 asymmetrical centre are oblong and small, and situated behind the former. 



The brain owes its apparent quadrilateral form to connective tissue, 

 but consists of two rounded ganglia. The external and superior angles 

 give off all the nerves to head and cephalic sense-organs. The inferior 

 external angles give origin to the connectives uniting the brain with 

 the pedals and with the first ganglia of the asymmetrical group. Where 

 the cerebro-pedal connective plunges into the pedal ganglion there arises 

 the very short connective uniting the latter to the asymmetrical ganglion. 

 The cerebral nerves are very closely apposed, the optic is almost always 

 distinct from the tentacular. The latter forms five ganglionic thicken- 



* Comptes Eendus, cv. (1887) pp. 978-82. 



