SPECIAL ANATOMY. 55 



and vice versa. In Cylindrella it reaches its maximum length and nar- 

 rowness ; in Succinea it has the minimum length, and the greatest pro- 

 portionate breadth. When the foot is protruded from the shell, every 

 part of the exterior surface of the turbinated mass is still in contact 

 with the interior surface of the latter, and is retained so by means of 

 the comparatively capacious pulmonary chamber. When the' foot is 

 retracted, it is at the expense of the latter cavity • so that the pulmo- 

 nary chamber of the testaceous genera is as much larger than that of 

 the naked genera as the size of the foot superadded, whilst the extent 

 of the pulmonary network of blood-vessels remains the same. 



The testacea have a muscle which is peculiar, namely, the retractor- 

 muscle of the foot, which has its origin, in common with the retractors 

 of the eye-peduncles and buccal body, from the columella of the shell. 

 Narrow at its commencement, it increases in breadth, splits into several 

 bands, and diverges as it descends to get its insertion into the whole of 

 the inner margin of the excavation of the foot, excepting anteriorly, 

 where its place is occupied by the retractor of the buccal body. 



The head occupies the anterior portion of the foot, and in Helix, JJu- 

 limus, Pupa, and Succinea, etc., offers nothing peculiar from that of 

 Limax. In Glandina a third pair of tentacular appendages exists. 

 These are non-retractile, auriculate in form, and originate just postero- 

 inferiorly to the base of the inferior, retractile tentacles, and project 

 horizontally backward. 



The body of the testacea, like that of slugs, has two great cavities. 

 The visceral cavity includes the greater part of the turbinated mass 

 and the excavation of the foot. The pulmonary chamber occupies a 

 position on the outer side of the lower one to three whorls of the turbi- 

 nated mass. The collar apparently takes the place of the mantle in 

 slugs. In all the genera it is attached around the base of the turbi- 

 nated mass, and is perforated on the right side by the pulmonary orifice. 

 On the outer border of the latter the anal aperture is placed. 



As in slugs, the genital orifice is situated on the right side of the 

 head, more or less posterior to the eye-peduncles in the respective 

 genera. 



On the Tegumentary Covering of the Terrestrial Gasteropoda. 



Besides a testa capable of enclosing the whole body, which most of 

 the terrestrial Gasteropoda possess, they have a thick envelope, com- 

 posed of mucous and muscular membrane. The exterior, highly irri- 



