THE BACKGROUND OF DARWINISM—ADAPTATIONS 199 
the rectum of the sea cucumber and may be seen to lie 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 
harmisdone. Another case 
of this sort is that of several 
species of Remora which 
attach themselves by a large 
diskoid adaptation on top of Fic. 41.—Fierasfer acus, penetrating the 
the head to various fish such anal openings of holothurians, 2 natural size. 
(From Boulenger, after 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), 
