3o8 CHARACTERS OF INVERTEBRATE ANIMALS 



muscle the soft upper part of the body in which a large part of 

 the viscera are contained will be exposed, and it will be noticed 

 that this visceral hump, as it has been called, is twisted behind 

 in correspondence with the twist in the shell. Skirting the 

 visceral hump is a flap, produced by a pulling out, so to speak, 

 of the body wall, and known as the mantle. In the Ormer it 

 is narrow for most of its extent, but is very well developed in 

 the part underlying the row of holes in the shell, where it roofs 

 in a large mantle-cavity, which has a long slit-like aperture above 

 and a wide opening in front about the head. That this mantle- 

 cavity should freely communicate with the exterior is very 

 necessary, for not only does it contain the breathing organs, but 

 the intestine and the kidneys open into it. A very small amount 

 of dissection reveals the presence of the breathing organs in 

 the form of two plume-like gills attached along their sides, and 

 having their tips pointing forwards. The projecting end of the 

 intestine will also be seen, and right at the back of the cavity 

 two small holes by which the kidneys open. 



Just behind the mantle-cavity the heart is situated, consisting 

 of a central ventricle, which pumps purified blood from the gills 

 through arteries which come off from it fore and aft, and of a 

 thin-walled auricle on either side. The ventricle is folded round 

 the intestine, a noteworthy peculiarity, though one not known 

 to have any physiological meaning. A heart like this, which 

 contains pure blood only, is said to be systemic, and it should be 

 noted how markedly it differs from the heart of an ordinary fish, 

 which contains impure blood only. The complex heart of a Bird 

 or Mammal is physiologically equivalent to both these varieties 

 of heart, for its right half receives impure blood and pumps it 

 to the breathing organs, while its left half is concerned with the 

 reception of pure blood from those organs, and the distribution 

 of the same to the general system. 



The digestive organs of the Ormer consist of a long digestive 

 tube with large glands opening into it, and including pharynx, 

 gullet, stomach, and intestine, the last, as already noted, ending 

 in the mantle-cavity. Particular interest attaches to the pharynx, 

 or buccal mass, which is partly modified into a complex rasping 

 organ (odontophore), characteristic of two great groups of the 

 Mollusca. It essentially consists of a rounded cushion rising from 

 the floor of the pharynx, over which is stretched from front to 



