276 READINGS IN BIOLOGICAL SCIENCE 



out and sends rootlike branches almost all over the crab's body, like a 

 malignant tumor. The unbranched part of the parasite is little more than a 

 sac of reproductive organs, and these produce eggs and sperms, which 

 unite to form larvae. By this time, the host is killed, and with the decay of 

 its body, the larvae escape into the sea water ready for a period of free life. 



Commensalism may be defined as an association in which two organisms 

 exist in close association without any positive detriment to either. In some 

 cases the claim is made that the association is mutually beneficial, but as a 

 rule the relation is relatively one-sided. 



Some of the most remarkable cases of commensalism are found in con- 

 nection with elaborate colonies of ants. In some cases two species of ants 

 live together in the relationship of master and slave. The master species is 

 unable to perform any of the ordinary duties of the colony, such as se- 

 curing food, taking care of the young, etc. In extreme cases the masters 

 are only soldiers, specialized for fighting and marauding, and cannot even 

 feed themselves unaided. The slave species would be able to carry on to 

 some extent if not captured, but thrives exceptionally well under the pro- 

 tection of the soldier species. 



One of the weirdest environments the world affords is the bottom of the 

 sea at great depths. There it is dark and cold and almost devoid of oxygen, 

 while the pressure is almost unbelievably high. Yet in these vast and for- 

 bidding abysses there dwell in apparent comfort representatives of most of 

 the animal phyla. We do not at all understand the nature of the adaptive 

 mechanism that enables these animals to withstand with their frail bodies 

 the steel-crushing pressures that prevail at all such depths. We do know, 

 however, how some of the deficiencies of the environment are made good 

 by these denizens of the deep. Thus many abysmal forms produce their 

 own light by means of phosphorescent organs placed at advantageous 

 points of their bodies. Not only fishes of the depths, but some mollusks 

 possess forms of artificial lighting equipment. 



Equally highly adaptive to life in a world of darkness are the strange 

 eyes of some of the abysmal fishes. Sometimes these eyes are enormously 

 large, and thus adapted to bring to the perception of the animal the weak 

 light of the depths, or again they may be modified still further in a strik- 

 ingly peculiar manner, each being drawn out into a cylinder and projecting 

 from the side of the head like a telescope. Such eyes are in fact not tele- 

 scopes, but are merely adaptations for concentrating the lights of low in- 

 tensity and making the environment visible. 



Other creatures of the darkness live strange lives in caves, such as the 

 Mammoth Cave of Kentucky. Most cave dwellers are blind or nearly so, 

 and usually have a pale and ghostlike appearance because of their lack of 

 pigment. AH grades of defective eyes are found, ranging from those that 

 are merely somewhat smaller than normal to those that remain deeply 

 imbedded in the head in a relatively undifferentiated state. It goes with- 



