44 



ESSENTIALS OF BIOLOGY 



invertebrates ; the muscle pulls inward the tentacle in the same 

 way as the finger of a glove can be turned " outside -in " ; blood 

 injected into the cavity of the tentacle reverses this motion (4). 

 Fig. 10, 5, represents a ciliary epithelium, where the separate 

 " hairs," or cilia, " lash " and so move the part covered by the 

 epithelium, and so on. 



I 



Artery 



Vein 



NervK 



Fioced 

 ,bonjs 



ervc 



Moving LlgcuneTvb 

 bone 



Adxiuctor 

 TTuzscle 



Shed 



Soint 



Ecctens 



vn 



or 



Fig. 10. 



I, Diagram of part of the limb of a vertebrate; 2, diagrammatic transverse section of a 

 bivalve mollusc; 3, a retractile tentacle fully expanded; 4, the same partially retracted; 

 5, a ciliary epithelium. 



i2h. Organs of Nutrition. The mechanisms just studied 

 procure the food, which is then digested in alimentary cavities, 

 intestines, stomachs, etc. * 



Food is taken into the alimentary cavity, which may be the 

 simple coelenteron of a Hydra (Fig. 5, 2), or the mouth, stomach 

 and intestine of a vertebrate. Enzymes prepared by the glands 

 digest the food. Blood-vessels circulating in the walls of the 

 alimentary cavity absorb the digested food-materials. But there 

 may be no alimentary cavity (as in a Tapeworm) and the animal 

 then simply absorbs food materials through its integument. In 

 such cases (which are usually those of parasites) the animal inhabits 

 such media (intestine of some host-animal, etc.) where there are 



