ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
177 
of the pericardial cells that the organ in question has an excretory- 
function, the author does not doubt that the cells are excretory. The 
flattened form is well adapted to the outpouring of fluid, and the student 
need only be reminded that there is pavement epithelium in the 
Malpighian bodies of the Vertebrate kidney. 
Another argument in support of this view is afforded by the fact that 
the gland is best developed in the Opisthobranchiata. These forms 
have generally a very large ciliated renal funnel, the colossal cilia of 
which are capable of producing a powerful stream, and so must effect 
great suction on the pericardial fluid. As no concretions are to bo 
observed in the cells, and as they do not stain when carmine is injected, 
it is probable that water alone is excreted by these glands. 
When we come to consider the morphology of the pericardial gland 
of the Mollusca, we observe that the organ is not always developed in 
the same place or in the same way. In Lamellibranchs the glands may 
take their origin from the pericardial investment of the auricle, and in 
others from the anterior angle of the pericardium. Among the Gastro- 
poda the organ is borne by the auricle in the Prosobranchiata, while in 
the Opisthobranchiata there are very various spots at which the organ 
may be developed, and the same is the case also with Ceplialopods. 
For Gastropods, as for Lamellibranchs, the oldest pericardial glands 
appear to be the atrial, and such are seen in the Prosobranchiata. The 
variety of positions occupied by the gland in Opisthobranchs is probably 
due to independent acquirement of these new positions, and such glands 
should be recognized as secondary. 
Everything seems to show that the pericardial gland is an important 
organ in Mollusca. The extent of its development may stand in inverse 
relation to that of the coelom, and it may have some relation to the 
quantity of water needed by the animal. 
Vision of Pulmonate Gastropods.* — M. V. Willem has made a 
number of observations on the vision of snails, slugs, and other pulmo- 
nates, which has led him to the following conclusions : — They have a 
well-developed tactile sense, and are able to detect slight shocks of the 
ground on which they are supported, and slight movements of the sur- 
rounding medium. The terrestrial forms see very badly, and direct 
themselves chiefly by means of their olfactory and tactile sensations. 
They have a confused image of large objects at the distance of about a 
centimetre, but they do not distinguish at all clearly the forms of objects 
beyond a distance of one or two millimetres. The aquatic Pulmonata 
do not see distinctly at any distance whatever. The Mollusca do not 
seem to have that special power of seeing movements which has been 
demonstrated in Arthropods. The reaction to light varies, different 
species of snails and slugs being some fond of, and others fearful of light. 
The dermatoptic powers vary in various species. 
5. Lamellibranchiata. 
Crystalline Style.f — Prof. F. E. Schulze does not believe that this 
consists of reserve-material, as Hazay and Hasloff have maintained. In 
fact, histological examination shows that it is an epithelial secretion, 
* Comptes Rendus, cxii (1891) pp. 247-8. 
t SB. Gesell. Naturf. Freunde, 1890, pp. 42-3. 
1891. 
N 
