ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
579 
secreting surface, and tlie gland becomes localized, while it is more 
sharply marked off in higher types. The gill, as has long been known, 
is only a continuation of the folds formed by the internal layer of the 
mantle ; in the interior of these folds there is a system of muscular 
fibres which may adduct or abduct the two folds, and so diminish or 
increase the blood-spaces. When differentiation is highest the different 
regions of a lamella are, in consequence of the localization of the different 
epithelial elements, either secretory (on the afferent edge), sensory (on 
the efferent edge), or merely respiratory. The organ of Spengel is 
clearly sensory in function ; it owes its form to the accumulation of 
neuro-epitlielial cells on a nerve which either arises from (Diotocard'a) 
or ends in (Neritidae, Valvata ) the branchial ganglion, and persists after 
the ganglion disappears. 
These elements are not different from those which are found in other 
pallial organs, and that of Spengel has only a general sensibility ; when 
the differentiation is greatest the large number of nervous and neuro- 
epithelial cells which are present, the aj)pearance of pigment-cells, and the 
localization of the groups of elements, show that sensibility has increased, 
but do not demonstrate whether the sensibility is tactile or olfactory. 
Compared with the results obtained as to the tissues of the Pul- 
monata, Opisthobranchiata, and Acephala, we may note certain points of 
remarkable agreement ; at the same time dermal gland-cells are wanting 
from the mantle of Prosobranchs, although present in the foot. 
With regard to the classification of the group, M. Bernard urges 
that the distinction between bipectinate and monopectinate gills is of 
capital importance, because it clearly agrees with the principal charac- 
teristics drawn from other organs, and particularly those recently 
investigated by M. Bouvier and M. R. Perrier. In other words, the 
groups Aspidobranchiata and Pectinibranchiata agree with the Dioto- 
cardia and the Monotocardia ; Valvata has a bipectinate gill, while most 
of its characters approximate it to the Taenioglossata. In the Patellidae, 
the gill of Tecfura, like its nervous system, inclines us to place it with 
the Diotocardia, but the heart and kidney would make us separate it 
from them. 
Among the Diotocardia, the classification, proposed by Spengel and 
adopted by Bouvier, into Zygobranchiata and Azygobranchiata does not 
appear to be satisfactory. The most natural classification seems to agree 
with that of R. Perrier, and in it we have the following divisions : — 
A. Scutibranchiata = Diotocardia = Aspidobranchiata = Rhipido- 
glossata. 
(1) Fissurellidae (Homonephridiata). 
(2) Trochidae, Turbonidie, Haliotidae, &c. (Heteronephridiata). 
(3) Neritidae (Mononephridiata = Orthoneuroidea). 
B. Cyclobrancliiata = Heterocardia = Doxoglossa. 
Patellidae, Tecturidae, Lepetidae. 
In the Monotocardia the false gill varies considerably, and its 
different degrees of complication have been utilized by M. Bouvier in 
his classification ; they agree with the characters drawn from the 
anterior part of the digestive tube and the nervous system. The author 
consequently proposes no change in the classification proposed by his 
predecessor. 
