338 CHARACTERS OF INVERTEBRATE ANIMALS 



serving to attach the creature to some firm object. There is in 

 this genus an interesting variation on the mode of opening the 

 shell described for the Fresh- water Mussel (see p. 330). There 

 is no external ligament but what is called an internal ligament 

 or cartilage, placed in a deep pit at the hinge and kept compressed 

 when the shell is shut. When therefore the adductor muscle 

 ceases to contract, the elasticity of this body will come into play, 

 causing the shell to gape, just as a door might be made to fly 

 open by the expansion of a piece of india-rubber shut into its 

 hinge and thereby strongly compressed. The mantle - lobes of 

 Pecten are quite free from one another, so that not only are 

 siphons absent but also special inhalent and exhalent openings. 

 The long plate-like gills follow the curve of the body, and water 

 has ready access to them through the wide cleft between the 

 mantle-lobes exposed when the animal opens its shell. The edge 

 of the mantle is fringed by long tentacles, and bears quite a 

 number of spherical eyes of a beautiful green colour. These have 

 a very complicated structure, approaching in some respects to the 

 eyes of Vertebrates. 



The Oyster Family is closely allied to the preceding, but its 

 members are still more modified. The shell is very irregular, 

 and the animal is attached by the substance of the left valve, 

 which in the Common Oyster (Ostrea edulis] of British seas is 

 hollowed out while the right valve is lid-like, just the opposite 

 to what is the case in a Scallop. The foot is entirely absent, 

 and though the mantle-edges bear short tentacles, they are devoid 

 of eyes. 



Brief mention may be made of the Nucula Family as repre- 

 senting a comparatively small order of bivalves in which the gills 

 present primitive characters. In the type genus Nucula, for 

 example, the gill on each side is small and obviously like one of 

 the gill-plumes of the Ormer, and a further primitive character 

 is found in the possession of a foot with flattened creeping surface. 



CLASS 4. TUSK-SHELLS (SCAPHOPODA) 



This small class includes the typical genus Dentalium and its 

 allies. A British form, the Common Tusk- Shell (Dentalium 

 vulgare) (fig. 199), may be taken as a type. It is found burrowing 

 in the sandy parts of the sea-floor. The curved body is bilaterally 



