MOLLUSCA 



123 



As in other Molluscan groups, we find even in closely- 

 allied genera (for instance, in Aplysia and Pleurobran- 

 chidium, and other genera observed by Lankester) the 

 greatest differences as to the amount of food-material by 

 which the egg-shell is encumbered. Some form their 

 Diblastula by emboly (fig. 7), others by epiboly (fig. 5) ; 

 and in the later history of the further development of the 

 enclosed cells (arch-enteron) very marked variations occur 

 in closely-allied forms, due to the influence of a greater or 

 less abundance of food-material mixed with the protoplasm 

 of the egg. 



Order 2 (of the Euthyneura). Pulmonata. 



Characters. Euthyneurous Anisopleurous Gastropoda, 

 probably derived from ancestral forms similar to the 

 Palliate Opisthobranchia by adaptation to a terrestrial life. 

 The ctenidium is atrophied, and the edge of the mantle-skirt 

 is fused to the dorsal integument by concrescence, except at 

 one point which forms the aperture of the mantle-chamber, 

 thus converted into a nearly closed sac. Air is admitted 

 to this sac for respiratory and hydrostatic purposes, and it 

 thus becomes a lung. An operculum is never present ; a 

 contrast being thus afforded with the operculate Pulmonate 

 Streptoneura (Cyclostoma, <fcc.), which differ in other 

 essential features of structure from the Pulmonata. The 

 Pulmonata are, like the other Euthyneura, hermaphrodite, 

 with elaborately-developed copulatory organs and accessory 

 glands. Like other Euthyneura, they have very numerous 

 small denticles on the lingual ribbon. The ancestral 

 Pulmonata appear to have retained both the right and the 

 left osphradia (Spengel's olfactory organs), since in some 

 (Planorbis, Auricularia) we find the single osphradium to 

 be that of the original left side, whilst in others (Limnaeus) 

 it is that of the original right side. 



In some Pulmonata (Snails) the foot is extended at right 

 angles to the visceral hump, which rises from it in the 

 form of a coil as in Streptoneura ; in others the visceral 

 hump is not elevated, but is extended with the foot, and 

 the shell is small or absent (Slugs). 



The Pulmonata are divided into two sub-orders according to the 

 position of the cephalic eyes. 



Sub-order 1. Basommatophora. 



Characters. Eyes placed mediad of the cephalic tentacles at their 

 base ; the embryonic velar area retained in adult life as a pair of 

 cephalic lobes (fig. 70, r) ; male and female generative apertures 

 separate, placed (as is typical in Anisopleura) on the right side of 

 the neck ; visceral hump well developed, with a well-developed 

 shell ; aquatic in habit 

 Family 1. Limnseidte. 



Genera: Limtueits, Lam. (figs. 3, 4, &c.); Chilinia, Gray; Physa, 



Draparn. ; Ancylus, Geoff. ; Planorbis, MulL, &c. 

 Family 2. Auriculidas. 



Genera: Auricula, Lam. ; Conomilus, Lam.; PitAarella, Wood. 

 &c. 



Sub-order Z. Stylommatophora. 



Characters. Eyes placed on the summit of two hollow tentacles ; 

 visceral hump well or not at all developed ; shell large and coiled, 

 or minute or absent ; almost exclusively terrestrial. 

 Family 1. Helicidas. 



Genera : Helix, L. (figs. 69, A; 72*) ; Vtirina, Draparn. ; Suc- 

 cinea, Draparn. ; Bulimus, Scopoli ; Achatina, Lam. ; Pupa, 

 Lam. ; Clausilia, Draparn., &e. 

 Family 2. Limacidse (Slugs). 



Genera : Umax, L. ; Incilaria, Benson ; Arian, Ferussac (fig. 

 69, D) ; Pannafdla, Cuvier ; Testacella, Cuvier (fig. 69, C), &c. 

 Family 3.Oncidiadie. 



Genera : Oncidium, Buchanan ; Peronia, Blainv. (fig. 72) ; 

 Vaginulus, Ferussac, &c. 



Further Remarks on Pulmonata. The land-snails and 

 slugs forming the group Pulmonata are widely distinguished 

 from a small set of terrestrial Azygobranchia, the Pneumo- 

 nochlamyda (see above), at one time associated with them 

 on account of their mantle-chamber being converted, as in 



Pulmonata, into a lung, and the ctenidium or branchial 

 plume aborted. The Pneumonochlamyda (represented in 

 England by the common genus Cyclostoma) have a twisted 



FIG. 69. A series of Stylommatophorons Pulmonata, showing transitional forms 

 between snail and slug. 



A. Htlii pomalia (tram Keferstein). 



B. Htlicophanta brtvipes (from Keferstein, after Pfeiffer). 



C. Testacetla haliotidta (from Keferstein). 



D. Arion ater, the great Black Slug (from KefersteinX 



a, Shell in A, B, C, shell-sac (closed) in D ; 6, orifice leading into the 

 subpallial chamber (lung). 



visceral nerve-loop, an operculum on the foot, a complex 

 rhipidoglossate or taenioglossate radula, and are of distinct 

 sexes ; they are, in fact, Azygobranchiate Streptoneura. 

 The Pulmonata have a straight visceral nerve-loop, never 

 an operculum (even in the embryo), and a multidenticulate 



FIG. "p. A, B, C. Three views of Li'miuetu ftagnalis, in order to show the 

 persistence of the larval velar area r, as the circum-oral lobes of the adult, 

 m, mouth ; /, foot ; r, velar area, the margin r corresponding with the 

 ciliated band which demarcates the velar area or velum of the embryo Gas- 

 tropod (see fig. 4, D, E, F, H, I, t>X (Original.) 



radula, the teeth being equi-formal ; and they are hermaphro- 

 dite. Some Pulmonata (Limnaeus, ic.) live in fresh-waters 

 although breathing air. The remarkable discovery has 

 been made that in deep lakes such Limnaei do not breathe 

 air, but admit water to the lung-sac and live at the bottom. 

 The lung-sac serves undoubtedly as a hydrostatic apparatus 

 in the aquatic Pulmonata, as well as assisting respiration. 

 It is not improbable that here, and in other air-breathing 

 animals, the hydrostatic function was the primary one, and 

 the respiratory a later development. 



