1898.] NATURAL SCIENCES OF PHILADELPHIA. 225 



supposed it to be an aid to the extension of the penis. We scarcely 

 endorse this view of its function, after observing it in several other 

 genera in which it is not inserted near the atrium, but in the integ- 

 ument of the head or back above. It may aid in withdrawing the 

 head beneath the mantle. Morphologically this muscle is regarded 

 by Simroth as a slip detached from the body-wall. In this, again, 

 we are compelled to dissent. We regard it as a retractor pedis, the 

 anterior insertion of which has moved from the sole to the side or 

 upper integument. Pendiug further investigation into its physiolo- 

 gic function and morphologic equivalency, we continue to use Sim- 

 roth's name " retensor " for this muscle. 



The retractors of the genitalia are far less constant in form, num- 

 ber and position than those of the buccal mass and eyes. Their 

 proximal insertions here, as in the spiral snails, are on the diaphragm 

 or lung floor. The chief muscles of this system are the penis retrac- 

 tor (PI. XIV, figs. 66, 71 r.p.), which is almost invariably inserted 

 on the left side of the diaphragm, and the oviduct retractors (PI. 

 XIV, fig. 67, vag. r.), which may be inserted either upon the 

 diaphragm or at its posterior border. The latter are well developed 

 only in those genera in which the oviduct or vagina has assumed 

 the function of a penis. In Avion and Geomalacus the sperm atheca 

 duct also has a retractor. The several retractors of the female or- 

 gans are quite inconstant in position and number (being often in- 

 creased by splitting). They are not homologous with the penis re- 

 tractor, but have been developed de novo in the genera requiring 

 them. 



Another muscle deserving mention is a short band uniting the 

 swollen epiphallus to its peduncle, in the genus Prophymon (PI. 

 XIII, fig. 57, muse). This muscle so conceals the true structure of 

 base of the epiphallus that no former authors with the exception of 

 Simroth, have correctly described or represented it. 



Pallial region. — Underlying 9 the retractor muscle system is seen 

 a thin but dense membrane perforated only by the aorta and the 

 rectum. This is the " diaphragm " separating the body cavity from 

 that of the lung (indicated in outline in the figures on PI. XV). 

 Removing the diaphragm, the inner surface of the lung is exposed, 

 densely reticulated with blood vessels (PI. XV, fig. 73). The breath- 

 ing pore or pulmonary aperture (PI. XV, fig. 77, p. a.) is situated at 



9 The slug being pinned back downward in the dissecting pan. In the 

 natural position of the animal it overlies the muscles. 



