THE BACKGROUND OF DARWINISM— ADAPTATIONS 



199 



the rectum of the sea cucumber and may be seen to He with only its 

 head out. From this shelter it darts forth to capture its prey; which 

 done, it returns to its shelter. 

 Curiously enough the vent 

 of the little fish is situated 

 just back of its mouth so 

 that its wastes may be 

 voided when in its usual 

 position. There can be no 

 advantage to the sea cu- 

 cumber in such an arrange- 

 ment, though no particular ^,^p>^^ -idS#l?^- -- --^'-''^rvi'^B.' .• 



harm is done. Another case ^^^*K^ ; ^ : ^ T " "\' "" ^^^C "^ '-, 



of this sort is that of several sm^'-^ \ ' >....,„fic«;dU.iiri .-> . , ^'r^^ 



species of Remora which 

 attach themselves by a large 

 diskoid adaptation on top of Fig. 41. — Fierasfer acus, penetrating the 



the head to various fish such ^S""^ openings of holothurians, I natural size. 



{troni Boiiienger, ajtcr Emery.) 



as sharks, barracudas, etc. 



The sucking disk is a modified dorsal fin. The remora merely gains 

 free transportation to more favorable feeding-grounds. When the 

 desired food is sighted the passenger leaves its conveyance tempo- 

 rarily, but returns by a sudden swift dash and resumes its hold. 

 The shark gets nothing except perhaps the sense of companionship, 

 and is also undoubtedly somewhat hindered in its locomotion. 



Some of the most remarkable cases of commensalism are found in 

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

 of ants live together in the relationship of masters and slaves. The 

 master species is unable to perform any of the ordinary duties of the 

 colony, such as securing food, taking care of young, etc. In extreme 

 cases the masters are only soldiers, specialized for fighting and maraud- 

 ing, 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 protection of the soldier species. There 

 are among ants many varieties of commensal relationship less extreme 

 than this, but this will serve as a typical case. 



Communal life. — Among the higher insects and higher vertebrates, 

 especially among the ants and bees, we find a very elaborate social life. 

 In ants, for example, the typical colony consists of a queen (the only 

 fertile female in the colony), several males (mates of the queen). 



