ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
47 
Siphonaria.* — Dr. A. Kohler has made a close examination of this 
Mollusc, and tries to ascertain its exact systematic position, whether 
with the Pulmonata or Tectibranchiata. With regard to the gill of 
Siphonaria , it has been held by various authorities that (1) it is not 
homologous with the ctenidium or true Molluscan gill, but is a neomorph ; 
(2) it corresponds to a series of ctenidia, and that each of the “ gill- 
lamellae ” is homologous to a complete ctenidium ; (3) the whole gill of 
Siphonaria corresponds to a single ctenidium. 
The gill, as to which these very divergent views have been pro- 
pounded, consists of a series of folded plates which often carry secondary 
lamellae; these plates correspond exactly in structure and position to 
the gill-plates of Umbrella or Pleurobranchus ; they do not arise from an 
axis, but directly from the roof of the respiratory cavity, just as the 
anterior gill-plates of Umbrella arise from the side of the body. The 
opistliobranch Lobiger has, according to Mazzarelli, a pectiniform gill 
exactly like that of the young Siphonaria. While the gill of this 
Mollusc may, in its structure, be easily allied to the gill of Pleuro- 
branchidae, its position, on the other hand, is exactly that which obtains 
in the Tectibranchiata with well-developed respiratory cavity, e. g. 
Bulla. The differences in the arrangement of the organs which occupy 
the roof of the respiratory cavity in Siphonaria and in most of the 
Cephalaspideae appear to be chiefly due to the movement of the heart 
towards the left, and a prolongation of the gill towards the same side. 
The distance of the auricle from the gill is thereby increased, and the 
vessels that return the blood to the heart are broken up into two sets. 
The agreements between the gill of Siphonaria and those of Tecti- 
branchs seem to the author to show that we have to do with homologous 
organs, and this view is supported by the innervation of the parts. 
After a comparison with Umbrella , Dr. Kohler expresses his conviction 
that the gill of Siphonaria is a true ctenidium, and not a secondary 
neomorph due to adaptation to a special mode of life. From this it 
seems to follow that the genus in question is a true Opisthobranch and 
not a Pulmonate. Moreover, the nervous system of Siphonaria has a 
great resemblance to that of Umbrella , while the generative organs show 
points of affinity to the Bullidae. 
The Oncidiidae.j — In his second contribution to our knowledge of 
the Pulmonate Gastropods Dr. L. H. Plate discusses the characters of 
Oncidium and its allies. He is of opinion that the Oncidiidae form an 
aberrant side-branch of the stem-form of the Pulmonata, and are there- 
fore to be regarded as primitive forms, notwithstanding such secondary 
modification as the loss of the shell. From these primitive forms arose 
first the Basommatophora and later on the Stylommatophora. 
The author gives a number of reasons for regarding the Oncidiidae 
as Pulmonata and not Opisthobranchiata ; he urges that the cavity which 
serves for respiration is a true pulmonate lung-cavity in the morpho- 
logical sense, and brings forward evidence in support of this position. 
There is a close similarity between their central nervous system and 
that of the Limneae ; the generative organs are really of the Pulmonate 
type ; while, as in that group, the kidney has a lamellar strue^ure, and 
there is a similarity in position. There is a pedal gland in the 
* Zool. Jahrb. (Anat.), vii. (1893) pp. 1-92 (6 pis.), f Tom cit., pp. 93-234 (6 pis.). 
