THE DEGENERATION OF ARMOUR IN ANIMALS 127 



furnish in the slugs one of the most familiar instances of the 

 reduction of the shell. Thus, according to Simroth, 1 Vitrina 

 and Hyalinia of the Limacidae family are to be considered 

 ancestral types. On the one hand, Vitrina (fig. 1), in which 

 the shell is already insufficient to cover the animal when 

 retracted, seems to have given rise through the somewhat 

 flattened Daudebardia (Helicophanta) brevipes, fig. 2 (with the 

 mantle-lobes only partly reflected over the shell) to Parmacella, 

 fig. 3 (in which the flattened shell still shows a subspiral apex 

 open to the air), and thence onward to Limax, fig. 5 (with its 

 completely flattened, internal, disc-like shell). On the other 

 hand, the worm-eating Testacella, fig. 4 (still with an external 

 shell), has diverged in a less advanced degree from Daudebardia. 

 These forms are either wholly or occasionally carnivorous. 

 Their relationships are based mainly on a comparison of the 

 tooth-ribbons of the various genera. On the other hand, the 

 slugs Ariou (with no shell or only calcareous granules), 

 Geomalacus (shell unguiform), Oopelta (no shell) and Philomycus 

 (no shell) have been shown on similar grounds to display an 

 affinity to Helix. The Helicidan slugs reach their highest 

 differentiation in the naked Peronia and Onchidium (with dorsal 

 eyes on a vertebrate plan) and some of these forms seem to 

 be reverting to a marine mode of life. 



A similar degeneration of the shell, coupled with more 

 active habits and greater differentiation of form, is afforded by 

 the Opisthobranch section of the Euthyneura. In the Cretaceous 

 period, beds of limestone were often filled with the large, 

 massive shells of Actceonella (fig. 6) but at the present day their 

 near relatives — e.g. Aplustrniu, Atys, Philine (figs. 7, 8, 9) — are 

 so thin-walled as to be called bubble-shells (Bulloidea); they 

 are carnivorous and show the tendency to a more active and 

 pelagic life by being able to use their wide lateral mantle-lobes 

 for swimming. In some cases the body-whorl of the shell 

 becomes so large {e.g. in Philine aperta, fig. 9) that the shell is 

 widely open and in this case completely enclosed by the mantle- 

 lobes, which Garstang 2 has shown to be protectively coloured. 

 Indeed, a gradual series can be constructed of living species 

 of the Bulloidea, from Actmon to Doridium, clearly showing 

 how the reflected epipodia gradually assume the supporting 



1 Sitz.-Ber. Naturforsck. Ges. Leipzig, 1886-7, PP- 40-48. 



2 Conchologist, ii. p. 49. 



