164 THE STORY OF LIFE IN THE SEAS. 



network of stinging threads, and partly of the 

 mud in which the animal lives. 



The use of foreign inorganic substances for the 

 protection or concealment of animals is not, 

 strictly speaking, however, a part of the subject- 

 matter of this chapter, which was intended for the 

 consideration of the associations of two different 

 kinds of living organisms. 



The subject of Parasitism must now be con- 

 sidered, a subject which presents so many 

 features of interest that it is possible here only 

 to touch on a few points of general importance. 

 It is a well-known truism to say that Parasitism, 

 whether in human society or in animal life, leads 

 to degeneration ; but there are degrees of para- 

 sitism among animals, and consequently degrees 

 of degeneration exhibited by animal parasites. 

 We may roughly divide them into two classes, 

 the outside or skin parasites and the internal 

 parasites, the latter being invariably far more 

 modified in structure and in development than 

 the former. 



Among the terrestrial animals we find a great 

 number of external parasites, such as the Fleas 

 and the Bugs, which are only slightly modified, 

 as in the loss of their wings, owing to their 

 habits, and can live an active, if not a very 

 prosperous, life for some length of time apart 

 from the society of their hosts. There are 

 others, such as the Mosquitoes, Ticks, and 

 Leeches, which are only occasional parasites ; 

 that is to say, they will suck the blood of 

 another animal when the opportunity is pre- 

 sented, but failing that, are able to continue 



