322 Proceedings of the Royal Society of Edinburgh. [Sess. 
Limnaea to its passive migration, but Klotz (1889) denies that any such 
migration takes place. 
Yon Jhering and Brock assign to the penis a quite different mode of 
origin. The former (1875) states that the parts of the genital apparatus in 
Helix pomatia and H . nemoralis are not separately developed (differing 
from Eisig’s results on Limnaea), but arise as differentiations of a meso- 
dermal rudiment. Brock (1886) describes the first appearance of the penis 
in Agriolimax agrestis (L.), not as a separate structure but as a swelling, 
afterwards a blind sac, on the purely mesodermal primitive genital duct, 
which latter is not at first in connection either with the ovotestis or the 
exterior. He also found that the vas deferens develops as an outgrowth 
from the inner end of the penis, and its free end unites with and opens 
into the common duct. The genital duct establishes communication with 
the exterior by means of the genital atrium opening outwards : there is, 
according to Brock, no trace of invagination of the skin to form either the 
penis or the atrium. For several reasons it is unfortunate that Brock’s 
careful work was carried out on Agriolimax, which, however convenient it 
may be from the point of view of microtome technique, is so modified in 
various directions that it could scarcely be expected that the ontogeny of 
the genital organs would remain unaffected.* Especially from the point 
of view of the development of the penis, the particular species ( agrestis ) 
investigated was very ill adapted for the purpose, as Simroth (1887, pp. 
646-647) remarked, because of the indefinite form and structure of that 
organ in the adult. 
Subsequent to the publication of Brock’s results Klotz (1889) investigated 
the development of the sexual organs in Limnaea ovata and affirms that the 
penis arises independently at the hinder margin of the right tentacle, as an 
ectodermic invagination, thus closely agreeing with Eisig (who, however, 
described the ingrowth as solid). 
A consideration of the above accounts seems to indicate that in Limnaea 
the penis is formed directly from the epidermis either as a solid or a hollow 
ingrowth, but in Helix and Agriolimax its development is bound up with 
that of the genital atrium and the neighbouring parts of the female duct, so 
that the penis appears to arise from the atrium or from the oviduct, more 
probably from the former which is relatively long in the earlier stages. 
The vas deferens, according to Eisig and Brock, arises as an outgrowth 
from the inner end of the penis, but according to Rouzaud, Simroth and 
* See also the observations of Simroth and Babor below (p. 324), which show that the 
penis in some Limacidae, e.g. Agriolimax laevis and Limax maximus , is frequently delayed 
in development. 
