COMPARATIVE ANIMAL MORPHOLOGY AND PHYSIOLOGY. 311 



to increasing specialization, different cells performing different 

 functions. Tissues and cellular organs result. 



Amoeba is usually irregular in form, but, when at rest, tends to 

 become spherical. This is simply a physical result of its semi- 

 fluid nature. The firm cuticle possessed by Vorticella and 

 Gregarina necessitates a definite form. 



Bilateral symmetry (exhibited by most of the forms) may be 

 conceived of as derived from radial symmetry (seen in Hydra), 

 by elongation transverse to the long axis, which, at the same 

 time, shortens. Meanwhile, ventral and dorsal surfaces, and 

 anterior and posterior ends, become differentiated, as the result 

 of a creeping mode of progression. The anterior end ultimately 

 becomes the head, in which some of the most important organs 

 are located. 



The higher forms mostly exhibit metameric segmentation, 

 that is, a division from before backwards into segments or meta- 

 meres. These are similar in the Earthworm and Leech, but in 

 the Crayfish are more or less dissimilar, and largely fused. It is 

 thus that distinct body-regions arise. The Fluke and Ascaris are 

 unsegmented, and the Mussel and Snail have lost all traces of 

 segmentation. The last has also become twisted and asymmetrical 

 owing to the development of a spiral shell. Amphioxus is clearly 

 segmented but very unsymmetrical. Segmentation is obvious in 

 the Dogfish, but in the Frog, Pigeon, and Rabbit, is much 

 obscured in the adult. It is best seen in the spinal nerves. The 

 jointing of the backbone appears to be a case of " secondary " 

 segmentation i.e., the segments do not correspond to the primi- 

 tive ones indicated by the mesodermic somites (protovertebrae) 

 of the embryo. 



Homology and Analogy are important kinds of agreement. Homo- 

 logous organs agree in relative position and mode of development, analogous 

 organs correspond merely in function. For example, the quadrate bone of 

 the Pigeon is probably homologous with the incus of the Rabbit, though 

 their functions are different, while the breathing-organs of the Snail, Cray- 

 fish, and Rabbit are analogous, agreeing in function. Homology is, in fact, 

 morphological, analogy physiological equivalence. It must not be forgotten 

 that organs are often both homologous and analogous e.g., the eyes of 

 Dogfish, Frog, Pigeon, and Rabbit. 



Serial homology is agreement in plan of structure and mode of develop- 

 ment between series of structures e.g., the segments of the Earthworm, or 

 spinal nerves and vertebrae of the Frog. 



1. Protection and Support. In Amoeba there is, under ordinary 



