596 SUMMARY OF CURRENT RESEARCHES RELATING TO 
Vision of Gastropoda.* — Dr. 0 . Willem is led by bis experiments 
to tbe conclusion that terrestrial pulmonate Gastropoda see very badly, 
and that they direct themselves principally by means of their olfactory 
and tactile sensations ; they perceive a confused image of large objects 
at a distance of about a centimetre, but they do not get a distinct image 
of objects that are more than one or two millimetres off. Aquatic 
Pulmonates do not seem to have any distinct vision at any distance. As 
to moving objects, the terrestrial forms seem to distinguish them worse 
than fixed objects, and they appear to make no impression on aquatic 
forms. As to the effect of light, some seek it and some shun it ; the 
amount of reaction differs in different species, but for a given species 
the reaction is in proportion to the intensity of the light. They have 
dermatoptic sensations, but the extent to which different species are 
affected varies considerably. They do not seem to be able to perceive 
ultra-violet rays. 
Aquatic branchiate Gastropods do not appear to be able to distinguish 
form at all. The author has some notes on the structure of the eye in 
various Prosobranchs and Opisthobranchs, and points out that a retro- 
gression in the structural characters of the eye often accompanies the 
migration of the eye into the tissue. Diminution may be seen in the 
size of the precorneal lacuna, the volume of the eye, and the number of 
retinal cells. 
Morphology of Liver of Gastropoda.f — M. H. Fischer has made a 
study of the liver of Gastropods. In its development he distinguishes 
several stages. The hepatic tissue is preceded by a reserve-tissue which 
is formed by part of the endoderm. In typical cases ( Paludina , 
Sphserium ) it is the cells that are situated on the ventral wall of the 
arclienteron which form the tissue. This transitory stage corresponds to 
the conformation of the digestive tube in adult Solenogastres, as described 
by Pruvot. The ciliated cells are localized on the dorsal median line, 
where they form the first rudiment of the stomach ; on the ventral 
surface and on either side the walls are formed by the hepatic cells. 
The stomach is completed by the appearance of cylindrical cells on 
the median line of the ventral surface. At this stage the median region 
of the digestive tube consists of three parts ; the median stomach and 
two endodermic diverticula which open laterally into the stomach by a 
pair of orifices which gradually close. When the larvae escape the 
nutrient reserve-substances are used up, and the hepatic lobes function 
as digestive organs ( JEolis ). 
The hepatic lobes, in the third stage, are at first rounded ; they 
break up into lobules and the liver is rapidly formed. 
In discussing the principal variations of the liver M. Fischer deals 
first with the symmetry of the hepatic lobes. The two lobes of the 
embryo are equal and symmetrical in the Scutibranchiata ( Neritina ), 
and many other organs of these animals present the same peculiarity. 
Among the Pectinibranchiata, Valvata alone has two symmetrical lobes, 
and it, like the Scutibranchiata, has a bipectinate gill. In all other 
Gastropods the two endodermic diverticula are unequal from the first 
* Arch, de Biol., xii. (1892) pp. 57-149 (1 pi.). 
t Bull. Sci. de la France et de la Belgique, xsiv. (1892) 87 pp. and 6 pis. 
