Pleurobeanchiata.- 



-JIOLLUSCA. Aplvsiad.e. 



349 



Order I.— PLEUROBRANCHIATA. 



This orJcr contains eleven families, but, witli tlie 

 exception of tlie Ijiibble-sliells, none very nmnerons 

 in species. Tlie shell is spiral ; and both larvK and 

 adult are sliell-bearing. The \av\x are furnished witli 

 deciduous cephalic tins. The animals have the foot 

 elongate, adapted for walking. 



The old genus Bulla of Lamarck is now split into 

 man}', and even forms several families. The animals 

 are all characterized, however, by having the tentacles 

 broad, forming a more or less expanded frontal disk. 

 As Mr. Hancock observes, this frontal disk is in fact 

 nothing else than the dorsal and labial tentacles fused 

 into one continuous mass. They are all carnivorous 

 in their habits, swallowing their prey entire ; and their 

 stomach is furnished with a muscular gizzard armed 

 with calcareous plates. A fuller description of this 

 organ will be found under the genus Scaphander. They 

 have no operculum. 



Family— PIIILINID^. 



Some of the species of this family have no shells ; 

 olliers have a thin, fragile, ovate shell, which is con- 

 volute and covered by the mantle. 



Genus Scaphander. — See Plate 4, fig. 1 {Sca- 

 pJtander lignarius, shell and animal). This genus lias a 

 distinct shell, not covered by the mantle. It is ovate, 

 convolute, and spirally striated ; the spire is concealed, 

 and the aperture is large, much expanded in front, and 

 narrow behind. The animals are blind, and the giz- 

 zard is formed of two large trigonal calcareous plates, 

 and a small, narrow, transverse plate. This gizzard 

 was described by Gibeni as a genus of shells ! The 

 l!cv. l\Ir. Landsborough has described it very well in 

 the Scaplamdcr lirjiuiriiis, at the same time giving an 

 account of the habits of this species, which is found on 

 many parts of our sea-coasts. " This gizzard," he 

 says, " is a wonderful piece of mechanism, which one 

 would not e.xpect to find in the interior of a very soft 

 mollusc. I thought it was composed of two plates, 

 but I found there were three. Two of the plates are 

 triangular, and placed one above tlie other, like the 

 upper and nether millstones. They are quite flat, 

 Imt a little concave externally, and rather convex 

 internally ; they are bound together by strong carti- 

 lage, and on one of the sides of the triangle there is a 

 third valve or plate, giving strength to the cartilage, 

 and keeping the two grinders at some distance, except 

 at the centre, where the convex points meet, and thus 

 leaving, except at these points, room for the reception 

 of food in the triangular space between the two mill- 

 stones. The food of the Puilla (Sniphandcr) seems to 

 be the fry of other shell-fish. Though they seem to 

 indulge very freely as to quantity, they appear to be 

 wiser than our biped gourmands, for they keep to one 

 dish. In every one of the specimens I procured, the 

 capacious gullet was filled with the fry of Mactra 

 suhtruncala. The gullet was in the form of a corn- 



sack, quite distended ; for cacli contained some scores 

 of these little bivalves in an unbroken state. The 

 sack, however, gradually emptied itself into the giz- 

 zard ; and in this shelly mill the shells and their con- 

 tents were reduced to powder, or rather fine paste, 

 well fitted, we doubt not, to be wholesome nutriment 

 for the industrious little marine miller." 



Family— EULLID^E {True DiMlc-shdh.) 



This family as now restricted consists of shells which 

 are involute, exposed or external, or only partially 

 covered by the reflexed lateral lobes of the foot. 



Genus Bulla. — In this genus the shell is ovate or 

 snbglobose, convoluted, solid, smooth, and variously 

 coloured or mottled ; the spire is small, sunk, or hid- 

 den, causing the apex to be perforated, and the aper- 

 ture is longer than the shell, and rounded at each end. 

 The animal has a large lobe or disk, truncated in front, 

 bilobed behind, the eye immersed, subcentral, and the 

 foot short and subquadrate. — See Plate 4, fig. ] 

 {Akera carnosa). The gizzard is furnished with three 

 plates. The species are rather numerous, about fifty 

 having been described. In their habits they are 

 recorded as being found on sandy mud flats, slimy 

 banks in estuaries, and in brackish water near the 

 sea. At low water they may occasionally be seen 

 concealing themselves in the mud or under sea-weed, 

 where they have been observed exuding large quanti- 

 ties of mucus to maintain the moisture of their skin. — 

 {Adams.') Their food consists of small bivalve shells, 

 which they are able to crush and triturate by means of 

 their powerful gizzards. 



Family— AMPLUSTRIDiE. 



The species of this family have an oval, ventricose 

 shell, ornamented with highly-coloured bands or mark- 

 ings. The s]>ire is generally conical and blunt, the 

 aperture oblong or ovate, and more or less channeled 

 in front. 



Family— APLYSirOiE {Sea-hares). 



The Sea-hares have the shell infernal, covered by 

 the mantle, or it is wanting altogether. The animals 

 are slug- like in form; have the head produced, and 

 their ear-like tentacles separate, and slit externally. 

 The foot is long, drawn out into a tail behind, and las 

 large side lobes reflected over the back. The gizzard 

 is armed with horny spines. 



Genus Dolaisella.- In the animals of this genus 

 the side lobes of the foot are not expanded so as to 

 enable the creature to swim, and the shell is hard, cal- 

 careous, of a triangular shape, and with a curved and 

 callous apex. See Plate 4, figs. 3, 4 {DohheUa Rum- 

 j'hii). 



Genus Aplysia.— In the animals of this genus the 



