224 PROCEEDINGS OF THE ACADEMY OF [1898. 



saon is not hollow like a penis, but has a small duct only, the walls 

 being solid and muscular. 



The remaining organs do not differ from their usual structure in 

 Aulacopod pulmonates, although the position of the ovotestis is 

 varied in the several genera. 



Muscles. — Only the muscles lying free in the body cavity, not 

 those composing the external walls or sole, will be considered herein, 

 the latter being morphologically similar in Arionidce to the other 

 land snails. 



The free muscles belong to two groups : (1) retractors of the buc- 

 cal mass, eye-peduncles and tentacles, and (2) retractors of the 

 generative organs. 



The buccal and tentacle retractors, with the foot retractor or re- 

 tractors in ordinary spiral-shelled snails converge and are attached 

 posteriorly or proximally to the columella of the shell. In Limacid 

 or Arionid slugs the degenerate shell no longer serves as a support 

 for these muscles, which are inserted at or near the posterior margin 

 of the diaphragm, or floor of the lung ; this position approximating 

 pretty closely to the posterior or former columellar margin of the 

 vestigial shell. In the more primitive genera, Binneya, Hemphillia, 

 Ariolimax, etc., the retractors still converge to a point near the 

 middle of the back margin of the lung, under the posterior edge of 

 the shell (PI. XIV, figs. 65, 66, 67, 69, 71). In the more divergent 

 genera Avion (PI. XIV, fig. 72) and Prophysaon (PI. XIV, fig. 70), 

 the eye and tentacle retractors have moved from the middle to the 

 outer posterior angles of the diaphragm. The convergent retractors 

 of Ariolimax and its allies are a heritage from the spiral-shelled an- 

 cestors of the family, although the utility of the convergence is no 

 longer present ; while the parallel retractors of Avion, etc., are a 

 later modification which resulted in a straight backward pull of 

 each retractor, independent of the others, and possibly brought about 

 mechanically by the tendency toward separation of the grouped 

 proximal insertions by strains on the converging muscles from their 

 separated distal terminations. Another muscle more or less closely 

 associated with the buccal and tentacle retractor system, is found in 

 Ariolimax, Hemphillia and allied genera (PI. XIV, figs. 65, 66, 69). 

 This is a band passing from near the proximal insertions of buccal 

 and eye retractors forward across the diaphragm to an insertion in 

 the top or right side of the head. It has been termed the " reten- 

 sor " by Simroth, who observed it in Ariolimax Calijornicus, and 



