334 CHARACTERS OF INVERTEBRATE ANIMALS 



characters of the gills, and embrace between them some forty 

 families. It will be enough for our present purpose to mention 

 a few common species in illustration of the range of characters 

 found within the class. 



The Cockle Family includes a large number of shallow-water 

 forms found along coasts and estuaries in most parts of the 

 world. They are especially characteristic of places where sand 

 is abundant. The best-known British form is the Edible Cockle 

 {Cardium edule) of the Atlantic and Mediterranean areas. The 

 shell is rounded in outline, and marked by prominent ribs which 

 radiate from the beak. The edges of the mantle - lobes are 

 much more extensively united than is the case with the Fresh- 

 water Mussel, but an orifice is left in front through which the 

 foot can be protruded, and at the back inhalent and exhalent 

 apertures are seen as before, a striking difference being that they 

 are here placed at the ends of two short tubes or siphons, both 

 fringed by tentacles. The narrow bent foot is able not only to 

 push the animal through the sand, but also by its sudden con- 

 traction to bring about springing movements in the water. The 

 siphons can be drawn back within the shell by means of a special 

 retractor muscle, and the attachment of this to the shell causes 

 the pallial line to be indented at its posterior end, just as, to use 

 a somewhat fanciful comparison that has been employed, a coast- 

 line is broken by a bay or, to use the Latin 

 word, sinus. By examination of the shell only 

 we are therefore able to say in a given case 

 (fig. 193) whether siphons of any size were 

 present, the extent of the bay being roughly 

 va^'f of'f'ri^upaiiiate shdL proportional to their development. This sinu- 

 IL^e^ssIeno^tU"riS.7'''''''' pilHate condition is contrasted with the integro- 

 palliate one described for the Mussel (p. 331), 

 but it must not be forgotten that small siphons may be present 

 devoid of muscles sufficiently powerful to indent the pallial line 

 perceptibly. 



The Gaper Family presents a certain amount of resemblance 

 to the preceding as regards habit, and a common British form, 

 the Sand Gaper [Mya arenaria) (fig. 194), is found both in mud 

 and sand off many parts of our coast, and also on the opposite 

 side of the North Atlantic. The thick oval shell is not ribbed 

 like the Cockle, and the name "Gaper" has reference to the 



