. Readings in evolution, genetics, and eugenics. Evolution; Heredity; Eugenics. THE BACKGROUND OF DARWINISM—ADAPTATIONS 199. Fig. 41.—Fierasfer acus, penetrating the anal openings of holothurians, f natural size. {From Boulenger, after Emery.) 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- cum
. Readings in evolution, genetics, and eugenics. Evolution; Heredity; Eugenics. THE BACKGROUND OF DARWINISM—ADAPTATIONS 199. Fig. 41.—Fierasfer acus, penetrating the anal openings of holothurians, f natural size. {From Boulenger, after Emery.) 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 harm is done. Another case of this sort is that of several species of Remora which attach themselves by a large diskoid adaptation on top of the head to various fish such 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.—A
Size: 1625px × 1537px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, bookpublisherchica, bookyear1921
