MORPHOLOGY OF ANIMALS 



become branched or plume-like or may fuse together into 

 plates (Lamellibranchia). Gills are situated on those parts 

 of the body where they will be most exposed to currents of 

 water, and they occur in the most extraordinarily different 

 positions in different phyla; thus they may be found on 

 the limbs (some annelids, crustaceans, Fig. 13, B), on or 

 around the head (sedentary annelids, molluscoids), along 

 the sides of the body (primitive mollusks), on the lateral 

 walls of the pharynx (chordates), or as outgrowths of 

 the hind-gut (holothurians). Homology being "corre- 

 spondence in the relative position and connection of 

 parts," there can of course be no homology between struc- 

 tures occurring in such diverse positions, and yet within a 

 given phylum they may be homologous and of high mor- 

 phological value (for example chordates). 



In the chordates a series of gill-clefts opens right and 

 left through the walls of the pharynx (Fig. 12, B), and 

 in the lower classes of this phylum the gills are found as 

 highly vascular plates or tufts on the outer side of the 

 arches lying between these clefts; water is taken in 

 through the mouth and then forced out through the gill- 

 clefts and thus over the gills. In the higher classes of the 

 phylum (reptiles, birds, and mammals), imperforate gill- 

 clefts and gill-arches are present during embryonic life, 

 though at no time in their entire life history do these 

 animals have gill filaments and respire water. The con- 

 stancy of gill-clefts and arches among vertebrates gives 

 this character a high value in determining the affinities of 

 such doubtful forms as Balanoglossus, Cephalodiscus, and 

 Tunicata. 



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