578 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
of the Philippines and Mauritius, and adds for purposes of comparison 
a description of Nerita jpeloronta and Neritelln jpulligera. 
Pallial Organs of Prosobranchiata.* — M. F. Bernard has a lengthy 
memoir on the pallial organs of the Pros jbranchiata. After some intro- 
ductory chapters he discusses in detail the organ of Spengel ; first 
in Cassidaria, where differentiation has reached a maximum ; the 
progressive differentiation in the Diotocardia is traced through 
the Neritidse, Fissurellidae, Trochidae, and Haliotidae ; similarly the 
Monotocardia are described — the Valvatidee, the Littorinidse, the 
Vermetidae, and others, the Naticidse, the Siphonostomata proboscidifera, 
the Rachiglossata, the Cypraeidae, and the Toxoglossata, being taken in 
order ; next Eelicina and Cyclophora, Prosobranchs without the organ of 
Spengel, and then the Patellidae are discussed. The organ of Lacaze 
Duthiers in the Pulmonata, Paludina, and the Opisthobranchs are the 
subjects of the next three chapters. The author, in his third part, deals 
with the structure of the branchial lamellae, making a special study of 
the muscular elements and the interepithelial nervous plexus. Finally, 
with the mucous gland a study is made of secreting elements. Morpho- 
logical and histological comparisons of the neuroepithelial cells in Gastro- 
pods and Acephala, the morphology of the venous system, the connec- 
tive tissue and blood-spaces form the subject of more general chapters. 
We have only space to deal with the more general conclusions at 
which M. Bernard arrives. Noth withstanding the numerous variations 
presented by the pallial organs in the different types examined, it is 
possible to show that not only are the homologous organs composed of 
the same elements, however different their morphological differentiation, 
but also that, in a general way, these elements always belong to the 
same types, whatever organs are considered. 
Thus, there are three types of epithelial cells — the secretory, the 
indifferent (which is generally ciliated in Prosobranchs), and the sensory. 
The connective elements are of four kinds — multipolar, plasmatic, endo- 
thelial, and greatly elongated cell-fibres. The nervous elements do not 
differ from the two forms described for other organs — these are multipolar 
ganglion cells with prolongations, some of which are much more important 
than others, and nerve-fibres with proper nuclei. This result is of special 
interest, as it applies to the nervous plexuses found in the epithelium, 
and hitherto incompletely known. The muscular elements are very fre- 
quently branched ; they form long narrow bands or short trabeculae with 
numerous prolongations, which connect two adjoining connective plates. 
All these elements are, as a rule, found in all parts of the mantle, 
and in all the pallial organs, whatever be their degree of differentiation. 
The cause of the differentiation of an organ or its functional specializa- 
tion is the accumulation of certain elements of each of these categories. 
For example, the three varieties of epithelial cells exist normally in 
the mantle, but in the region between the rectum and the gill the glan- 
dular cells are much more abundant than elsewhere, and the region 
becomes specially secretory. A simple modification of the epithelium 
brings about this transformation. The accumulation of glandular 
elements is correlated with the formation of folds which increase the 
* Ann. Sci. Nat., ix. (1890) pp. 89 -304 (10 pis.). 
