XXXII.—A Specimen of Helix pomatia with Paired Male Organs
Authors: Ashworth J. H. 
Published: June 2016

1. An account is given of a fully grown specimen of Helix pomatia, which, in addition to the normal set of reproductive organs present on the right side, possesses also on the left side a set of accessory male organs—namely, a penis and its retractor muscle, vas deferens, and flagellum. There is no trace of connection between these supernumerary organs and the ovotestis or the genital ducts of the right side.

2. The normal and supernumerary penes are equally developed and are symmetrically placed with regard to the median plane of the head.

3. The extra penis has the usual structure; it possesses a fully-developed copulatory organ and a muscular tubular introvert upon the inner sheath.

4. The extra vas deferens opens at one end into the penis and at the other terminates blindly in the sub-epidermal tissue; the blind end is bound to the epidermis by a short cord of connective tissue.

5. The extra flagellum is normal in size and in its internal structure.

6. The penial nerves on both sides have a corresponding course from the cerebral ganglion to their distribution on the penes. Owing to defective preservation it was impossible to determine whether the fibres of the extra penial nerve arise from the ocular nerve, as is apparently the case, or whether they really arise from the pedal ganglion, as do those of the normal penial nerve.

7. The penis opens into a short atrium which communicates with the exterior by a genital aperture of normal size and shape. This supernumerary genital aperture occupies a position on the left side exactly corresponding to that of the normal one on the right side. About 2·5 mm. posterior and slightly dorsal to it is the blind end of the vas deferens, which is situated close to the dorsal margin of the epidermal groove, which traverses the anterior portion of the animal.

8. In the duplication of the penis and its associated structures this snail presents an example of lateral homoeosis. The only comparable cases recorded in the Mollusca are a Pteropod with paired penes (see Addendum, p. 327) and a specimen of Moschites cirrosa, in which not only was the third right arm hectocotylised as usual, but the third left one also.

9. The supplementary vas deferens and the retracted penis form a U-shaped loop, both ends of which are in contact with the epidermis, and so closely resemble the corresponding structures figured by Plate in the primitive Pulmonate, Pythia scarabeus, that these organs may be described as having identical relations in the two forms, except that in the supplementary organs of Helix the vas deferens has no actual opening at its epidermal end and that it bears a flagellum which is not present in Pythia. It is suggested that the supplementary organs of Helix, having been able to develop free from the disturbing influence of the vagina, oviduct, and their accessory structures (which on the right side have been secondarily moved forwards in phylogeny), have assumed a condition closely resembling that which they would present in the ancestral form in which male and female apertures were some distance apart. The form of the extra organs (the vas deferens having a close connection at one end with the epidermis), which, it is suggested, might also be assumed by the corresponding normal organs if they were also free to develop independently of the female structures, supports the view that the present condition of the genital ducts in Helix and in other Stylommatophora, has been derived from a condition existing in the ancestral form in which the vas deferens and penis were connected with the primitive genital opening by means of a lateral groove, such as is still found in Pythia.

10. Consideration of this and of other abnormalities which have been described in the Pulmonata leads to the conclusion that the penis develops as an epidermal structure which in Helix and other Stylommatophora is closely associated with, and is secondarily carried inwards by, the invaginating atrium. In proterogynously hermaphrodite forms, especially in those in which the shell is depressed, reduced, or absent, the development of the penis rudiment may be postponed until the male elements in the gonad are further developed.