188 NATURAL HISTORY OF OUR SHORES 



halant " aperture, and the upper one the " exhalant " 

 aperture. 



In such forms as close the shell completely that is, with 

 the entire margins of the valves in contact, as in the cockle, 

 oyster, etc. these apertures are simply fringed slits in the 

 mantle, but in such forms as My a, Lutraria, etc. (the so- 

 called " Gapers "), where there is a bending out of the 

 valves at the posterior end, the apertures are in the form 

 of long tubes, " syphons," sometimes separate and some- 

 times joined side to side, giving a figure 8 section if cut 

 across. 



In some lamellibranchi these syphons are enormously 

 developed for instance, in Teredo (the " Ship-worm ") 

 they form the bulk of the animal's anatomy, and this has 

 given rise to its popular name. 



In such forms as bury in the sand e.g. Lutraria and Mya 

 these syphons project to the surface, and enable the func- 

 tions of respiration and nutrition to be carried on while their 

 owner is out of harm's way six or eight inches below. The 

 syphons are to a large extent retractile, and the lightest 

 touch, or even the tremor of a footfall (while the tide is 

 out), causes their instant withdrawal. 



On the gravel beaches at La Rocque Point, Jersey, and at 

 Herm Island, numbers of jets of water may often be seen 

 thrown up by the syphons of Lutraria and Solen, thus be- 

 traying the presence of the molluscs. 



The water, in thus circulating through the chambers, 

 performs a variety of functions. In its passage from the 

 lower to the upper chamber it has to pass the mouth. Here 

 two little flaps (labial palps) arrest and carry into the 

 stomach any nutrient particles which it may hold in sus- 

 pension. 



In the breeding season the spermatozoids which have 

 been set free in the water by the males are brought into con- 

 tact with the ovaries of the females, and fertilise the ova. 



