348 GENERAL ZOOLOGY 



strongly curved claws. It assumes other postures when climb- 

 ing vertical stems or when asleep. It is a good swimmer but is 

 incapable of assuming a standing position on the ground. All 

 of its movements are extremely slow, and active means of defense 

 seem to be entirely lacking; its fur and hide and its habit of 

 rolling up into a tight ball serve as its principal protection. This 

 passive means of defense is heightened by the presence of algae 

 in the fur. This is made possible by the structure of the hairs, 

 which are distinctly fluted, with four ridges and three grooves 

 on each side, the grooves offering a lodging place for the algae. 

 The algae, which are said to be always present in the long hairs, 

 change color under different degrees of moisture. When the fur 

 is dry, the algae produce a dirty brown color pattern; when wet, 

 the* pattern turns green. Such a protective coloration would 

 seem to be of value against enemies flying overhead. There is 

 no reason to suppose that there is a mutual respiratory benefit 

 such as is present in Chlorohydra and algae. 



A common symbiotic condition exists in the alimentary canal 

 of vertebrates in the presence there of bacteria, which appear to 

 be not only harmless but actually beneficial to the host to the 

 extent that they aid in digestion of the host's food. The bac- 

 teria benefit by being provided with food and protection. 



Parasitism. — When the association between two different 

 kinds of organisms becomes such that one of them, the parasite, 

 receives food and protection from the other, the host, without 

 giving anything in return, the condition is known as parasitism. 

 The effect on the host is usually detrimental, and if fatal, the 

 death of the host as a rule takes place only after the parasite has 

 developed to the point where the host is no longer needed. Judg- 

 ing from the great frequency of the condition, parasitism must 

 be a very successful way of living and reproducing, though often 

 it is the most complicated. The adjustments necessitated by 

 this mode of existence have resulted in remarkable states of 

 structural and functional adaptations on the part of the parasite, 

 producing in many cases a distinct specificity in the host of 

 hosts invaded by the parasites. Parasites may occur in prac- 

 tically any organ of the host's body, but the most common 

 location is the alimentary canal. Taenia solium, the pork tape- 

 worm, is found as an adult in the human intestines where it 

 reaches a length of 4 to 10 ft. The head or scolex (Fig. 231) is 



