COMMENSALISM AND PARASITISM. 165 



their life and their race independently. It is 

 not surprising, however, that the terrestrial 

 Vertebrates should be thus subjected to the 

 attacks of these parasites, as their feathery or 

 hairy skin affords a shelter and a foothold, from 

 which the efforts of their hosts to dislodge them 

 are exercised in vain. 



The skin of Fish, although covered with over- 

 lapping scales, is smooth and slippery, and with 

 the rapid movement through the water many of 

 the forms of parasites of the types we meet on 

 land would, if they existed in the sea at all, 

 find a difficulty in securing an attachment. It 

 is, however, provided with another means of 

 defence against skin parasites, in the possession 

 of numerous mucous glands which keep the body 

 bathed with a slimy fluid. 



Everyone must have noticed the slime that 

 exudes from freshly killed Fish, and if the finger 

 be pressed along the skin it is possible to see the 

 openings of the glands as the slime is squeezed 

 out. In the majority of Fish the openings of 

 the glands are most easily seen on the jaws and 

 the flap of the gill cover. 



We must remember that the sea is in most 

 places teeming with the larvae of Worms, Bar- 

 nacles and Zoophytes, and the spores of Algse and 

 Fungi of various kinds. Logs of wood, the iron 

 supports of piers, and the bottoms of ships become 

 covered with various fixed forms of animal and 

 vegetable life when submerged in the sea-water 

 for even a few weeks. How is it, then, that the 

 bodies of the Fish are usually so clean and whole- 

 some 1 The answer to this question is probably 



