MOLLUSCA 



111 



between tte mass of the liver and the muscular foot. The 

 radula has 160 rows of teeth with twelve teeth in each row. 

 Two paira of salivary ducts, each leading from a saUvary 

 gland, open into the buccal chamber. The oesophagus leads 

 into a remarkable stomach, plaited like the manyplies of a 

 sheep, and after this the intestine takes a very large num- 

 ber of turns embedded in the yellow liver, until at last it 

 passes between the two renal sacs to the anal papilla. A 

 curious ridge (spiral 1 valve) which secretes a sUmy cord is 

 found upon the inner wall of the intestine. The general 

 structure of the MoUuscan intestine has not been suffi- 

 ciently investigated to render any comparison of this struc- 

 ture of Patella with that of other MoUusca possible. The 

 eyes of the Limpet (28) deserve mention as examples of 

 the most primitive kind of eye in the MoUuscan series. 

 They are found one on each cephalic tentacle, and are 

 simply minute open pits 

 or depressions of the 

 epidermis, the epidermic 

 cells lining them being 

 pigmented and connected 

 with nerves (compare fig. 

 118). 



The Limpet breeds 

 upon the southern Eng- 

 lish coast in the early 

 part of April, but its de- 

 velopment has not been 

 followed. It has simply 

 been traced as far as the 

 formation of a Diblastula 

 which acquires a ciliated 

 band, and becomes a 

 nearly spherical Trocho- 

 sphere. It is probable 

 that the Limpet takes 

 several years to attain 

 full growth, and during 

 that period it frequents 

 the same spot, which 

 becomes gradually sunk 

 below the surrounding 

 surface, especially if the 

 rock be carbonate of lime. 

 At low tide the Limpet 

 (being a strictly inter- 

 tidal organism) is ex- 

 posed to the air, and is 

 to be found upon its spot of fixation ; but when the water 

 again covers it, it (according to trustworthy observers) 

 quits its attachment and walks away in search of food 

 (minute encrusting algse), and then once more as the tide 

 falls returns to the identical spot, not an inch in diameter, 

 which belongs, as it were, to it. Several million Limpets 

 — twelve million in Berwickshire alone — are annually used 

 on the east coast of Britain as bait. 



Order 2. — ^Azygol)ranchia. 



Characters. — Streptoneura which, as a sequel to the 

 torsion of the visceral hump, have lost by atrophy the 

 originally left ctenidium and the originally left nephridium, 

 retaining the right ctenidium as a comb-like gill-plume to 

 the actual left of the rectimi, and the right nephridium 

 (that which is the smaller in the Zygobranchia) also to the 

 actual left of the rectum, between it and the gUl-plume. 

 The right olfactory organ only is retained, and may assume 

 the form of a comb-Uke ridge to the actual left of the 

 ctenidium or branchial plume. It has been erroneously 

 described as the second gill, and is known as the para^ 

 branchia. The rectum itself lies on the animal's right 



Fig. 



36. — Nervous system of Fiasnrella. pi, 

 pallial nerve ; p, pedal nerve ; A, aMomi- 

 nal ganglia in the Strcptoneurous visceral 

 commissure, witli supra- and sub-intestine 

 ganglion on each side ; B, buccal ganglia ; 

 C, Cf cerebral ganglia ; es, cerebral commis- 

 sure; 0, otocysts attached to the cerebro- 

 pedal connectives. (From Gegenbaur, after 

 Jhering.) 



shoulder. The presence of glandular plication of the surface 

 of the mantle-fiap (fig. 46, x) and an adrectal gland (purple- 

 gland, fig. 47, ffp) are frequently observed. The sexes are 

 always distinct; a special genital duct (oviduct or sperm 

 duct) unpaired is present, opening either by the side of the 

 anus or, in the males, on the right side of the neck in con- 

 nexion with a large penis. The shell is usually large and 

 spiral; often an operculum is developed on the upper sur- 

 face of the hinder part of the foot. The dentition of the 

 lingual ribbon is very varied. In most cases the visceral 

 hump and the foot increase along axes at right angles to 

 one another, so that the foot is extended far behind the 

 visceral hump in the ab-oral direction, whilst the visceral 

 hump is lofty and spirally twisted. 



This is a very large group, and is conveniently divided ' 

 into two sections, the Eeptantia and the Natantia. The 

 former, containing the immense majority of the group, 

 breaks up into three sub-orders, the Holochlamyda, Pneu- 

 monochlamyda, and Siphonochlamyda, characterized by the 

 presence or absence of a trough-Hke prolongation of the 

 margin of the mantle-flap, which conducts water to the 

 respiratory chamber (sub-paUial space where the gill, anus, 

 &c., are placed), and notches the mouth of the shell by 

 its presence, or again by adaptation to aerial respira- 

 tion. The sub-orders are divided into groups according to 

 the characters of the lingual dentition. In some Azygo- 

 branchia the mouth is placed at the end of a more or less 

 elongated snout or rostrum which is not capable of intro- 

 version (Eostrifera) ; in the others (Proboscidif era) the 

 rostrum is partly invaginated and is often of great length. 

 It is only everted when the animal is feeding, and is with- 

 drawn (introverted) by the action of special muscles ; the 

 over- worked term "proboscis" is applied to the retractile 

 form of snout. The term " introversible snout," or simply 

 "introvert," would be preferable. The presence or absence 

 of this arrangement does not seem to furnish so natural a 

 division of the Eeptant Azygobranchia as that afforded by 

 the characters of the mantle-skirt. 



Section a.—BEPTANTIA. 



Oha/racters. — Azygobranchia adapted to a creeping life ; foot either 

 wholly or only the mesopodium in the form of a creeping disc 



Sub-order 1. — Holochlamyda. 

 Characters. — Eeptant Azygobranchia with a simple margin to the 

 mantle-skirt, and,- accordingly, the lip of the shell unnotched ; 

 mostly Eosteifera [i.e., with a non-introversible snout), and vege- 

 tarian ; marine, brackish, fresh-water, terrestrial. 



a. Shipidoglossa (x.4 to 7.1.4 to 7.x). 



Family 1. — Troehidm. 

 Genera: Turbo, Lin.; Phasianella, Lam.; Imperator, Montf.; 

 Trochus,\Jai..; Rotella, Lam.; Euomphalus, Low. 

 FamUy 2. — Neritidse. 

 Genera : Nerita, L. ; Neritina, Lam. ; Pileolus, Low ; Ncmkella, 

 Lam. 

 Family 3. — Plev/rotomaridse. 

 Genera : PUurotomaria, Defr. ; Anaiomm, Montf. ; Stomatia, 

 Helbihg. 



/3. Ptenoglossa (x.O.x). 

 Family 4. — Scalaridas. 



Genus : Sealaria, Lam. 

 Family 5. — Janthinidse. 



Genera : Jawthina, Lam. (fig. 44) ; Eecluzia, Petit. 



y. Tssnioglossa [Z.l.Z). 

 Family 6. — GeriOvidse. 



Genera ; Cerithium, Brug. ; Potamides, Brong. ; Neriniea, Defr. 

 Family 7. — Melanidse. 



Genera : Melania, Lam. ; Melanopsis, Fer. ; Ancylotus, Lay. 

 Family 8. — Pyramidellidee. 



Genera : Pyramidella, Lam. ; Stylina, Flem. ; AclU, Loven. 

 Family 9. — TurritelUdse. 



Genera : Turritdla, Lam. ; Cascum, Flem. ; Vermetus. Adans. ; 

 SUiquaria, Brug. 

 Family 10. — Xenophoridse. 



Genus : Phorus, Montf. (fig. 39). 



