REPRODUCTION 



173 



sea-cucumbers, etc.), the Mollusca (cuttlefishes, squids, nautiloids, clams, 

 oysters, and snails), and the Arthropoda (lobsters, shrimps, crayfishes, 

 insects, and spiders). Even in some of the phyla in which examples of 

 asexual reproduction occur there are many species, indeed some entire 

 classes and orders, in which asexual reproduction is never found. The 

 method is, however, wide spread among animals, and in many species 

 (oligochsete worms. Hydra and the Protozoa) it may occur for many 

 generations without the intervention of the sexual method. From its 

 occurrence principally in the lower animals, and from the comparative 

 simplicity of the process, asexual reproduction must be regarded as the 

 primitive method of forming new individuals. 



@ 



Fig. 132. — Reproduction in the protozoan parasite, Cocddium schubergi. Stages 

 represented by II-VII and Xla-Xllc are intracellular; IV-VIII represent sporulation; 

 IX-XIII, development of gametes; XIV-XX, development of spores. 



Sexual Reproduction. — Sexual reproduction is a well-nigh universal 

 method of reproduction. The method is employed by representatives of 

 every phylum of animals and by many of them to the exclusion of the 

 asexual method. It is also used by the plants, except the bacteria. It is 

 true that sexual processes have not been observed for all known species 

 of animals, since there are hundreds of species whose life histories are still 

 unknown. In a great proportion of these species the inference by analogy 

 from well known related species is that they too employ the sexual method 

 of reproduction. A phenomenon of such wide-spread (almost universal) 

 occurrence must have considerable import. 



