HOMOLOGOUS STRUCTURES (CONTINUED)

There are many examples of homologies among animals and plants.  The limbs of vertebrates are not the only obvious homologies in the skeleton -- they are the rule rather than the exception.  The staunch anti-Darwinian Richard Owen enjoyed much success in identifying homologous bones in vertebrates -- finding, for all practical purposes, one-to-one correspondences for most bones in most species. 

The number of neck vertebrae is constant among mammals, for example.  The neck of a giraffe is made of seven bones, just as yours is, and just as a whale's is.  In the whale's neck the bones are exceedingly short, in the giraffe they are exceedingly long.  One has to wonder why a structure that basically won't bend is made of seven ridiculously short bones rather than one big one, or why a giraffe doesn't have more bones in a ten-foot neck than a person with a ten-centimeter neck.  Perhaps if they did they wouldn't have to splay their front legs to get a drink of water.  There appears to be some constraint upon the way mammal necks are built.

The feeding structures of bryozoans and brachiopods are also homologous.  The pictures below show the lophophore in an adult bryozoan (left -- labeled 'lo') and brachiopod (right -- labeled '1' -- The small curved bit near the center is also part of it).  The lophophore in both taxa is used for feeding, passing food particles along the cilia to the mouth, which is at its base (labeled 'm' on the bryozoan and '7' on the brach).  The two are shown at vastly different scales - the bryozoan would be about a millimeter long, the brachiopod about 2 centimeters.    The bryozoan is also shown without its tubular shell.  (Bryozoan from G.A. Kerkut, 1959, The Invertebrata.  Brachiopod from A. Williams, 1956, Cambridge Phil. Soc. Biol. Rev., 31:247-287.)

The 'foot' of all the molluscs is also homologous.  The pictures below show the shells of the three main types of molluscs with the foot projecting from it.  In the case of the molluscs, the similarity of the feet is not so obvious.  One has tiny eyes and is a crawling appendage (gastropod), another has not eyes and is a digging device (bivalve or pelecypod), and the third has big eyes, numerous tentacles, and a tube for jetting out water and propelling itself as it swims (cephalopod).  With the brachiopods and bryozoa the similarities in form alone lead us to realize the structures are homologous, but how do we know the molluscan feet are homologous?  (snail from Cox, 1960, Treatise of Inv. Paleo.; bivalve from Lefevre and Curtis, 1910, Bur. Fisheries Bull.; cephalopod from Tasch, 1973, Paleobiol. of the Invert., after Yonge??)

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