GASTEROPODA 565 



at the posterior end of the mantle cavity and the two kidneys on each 

 side of the anus. There is a well-marked visceral loop and the pedal 

 nerve centres have the form of long cords in which ganglion cells are 

 evenly distributed. The gonad has no ducts but the genital cells are 

 discharged into the right kidney. The radula has numerous marginal 

 teeth arranged in a fan-like manner (rhipidoglossate type). 



Fissurella, the keyhole limpet (Fig. 380 B), is so-called because of 

 the hole which perforates the mantle and the apex of the shell. It 

 possesses two equal ctenidia. The visceral hump and shell are com- 

 pletely uncoiled, but in other respects it resembles Haliotis and 

 possesses the same type of radula. 



Patella, the limpet (Fig. 380 A), represents a type of complete 

 adaptation to life on an exposed coast between tidemarks. Its conical 

 shell only shows coiling in its early stages and offers the minimum of 

 resistance to the waves. As in the above forms there is no operculum, 

 but the mollusc cannot be detached from rocks without using great 

 force, owing to the enormous power of the pallial muscles which press 

 the shell against the rock. The mantle cavity is restricted anteriorly 

 and the ctenidia have disappeared, though the osphradia connected 

 with them are present as minute yellow specks. But a secondary 

 mantle cavity extends all round between the foot and the mantle and 

 contains a series of folds which are known as pallial gills . In the re- 

 lated Acmaeidae there are various stages of the loss of the ctenidia 

 and their replacement by pallial gills. The enormously elongated 

 radula is composed of very strong teeth and there are a small number 

 of marginals (docoglossate type). This type of radula is suited for the 

 feeding habits of the limpet, which scrapes the crust of minute algae 

 off the surface of rocks. Limpets have a remarkable "homing " sense, 

 returning after excursions for food to the same spot, which may be 

 marked by a depression in the rock. 



Suborder MONOTOCARDIA 



Biiccinum, the whelk (Fig. 380 D), lives between low-water mark and 

 100 fathoms. It is active and carnivorous, feeding on living and dead 

 animals, which it grasps by means of its foot. It has a remarkable 

 and highly developed proboscis which can be retracted within a 

 proboscis sheath. The true mouth is situated at the end of the pro- 

 boscis. The radula (of the rachiglossate type) is used for rasping 

 away flesh, but it can even bore holes in the carapace of Crustacea. 

 There is only a single ctenidium with a single row of filaments. 

 This is the primitive right member of the pair, though situated on 

 the left of the mantle cavity. A very prominent organ is the bipectin- 

 ate osphradium, which is easily mistaken for a ctenidium. There is a 



