49° THE INVERTEBRATA 



Taenioglossa : radula normally with seven teeth in each row. 



Natica feeds on shell fish. Littorina, the periwinkle, 



amphibious. S tr ombus pvogressts by leaping. Paludina and 



Ampullaria, fresh water. 



This tribe also includes a pelagic section, the Heteropoda 



(Pterotrachea). The rest are called the Platypoda. 

 Toxiglossa: radula with two elongated teeth in each row; a 



poison gland. Conus (Fig. 362 D). 



^uhordev DIOTOCARDIA 



Haliotis, the ormer (Figs. 356 C, 365 A), is a greatly flattened gasteropod 

 which lives between tidemarks, as far north as the Channel Islands, 

 browsing on seaweed and eating all kinds of dead organic material. 

 It can move with considerable speed (5-6 yards a minute), but 

 adheres very firmly to stones. The mantle cavity is very spacious and 

 contains two ctenidia, the left being rather the larger, each with two 

 rows of filaments. The mantle has a slit which runs in the roof of the 

 mantle cavity, its position being shown by a row of holes in the shell 

 which serve for the escape of the exhalant current. The anus opens 

 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. 356 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. 356 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 

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

 and their replacement by pallial gills. The enormously elongated 



