GENERAL ZOOLOGY 



cells. The zygotes and their accompanying supphes of yolk cells are gathered 

 in the genital atrium, where numbers of them are enclosed in a shell or cocoon 

 secreted by numerous gland cells in the wall of the atrium. A stalk, con- 

 tinuous with the cocoon, is secreted through the genital opening and fastened 

 to the substratum; as the worm creeps away, the cocoon is drawn out of the 

 atrium and left supported by its stalk. Within the cocoon the zygotes develop 

 into embryos which engulf the yolk cells and use their substance as food dur- 

 ing further development. After several days or a few weeks, depending on 

 external conditions, the cocoon ruptures, and small juvenile worms emerge 

 (Fig. 11.7). Except for their smaller size and the absence of a reproductive 

 system, which develops later, these juveniles resemble the adults in most 

 respects. 



Although complete male and female systems are thus present in each in- 

 dividual, conditions which are not well understood apparently prevent self- 

 fertilization in most cases. Only a few species of fresh-water planarians are 

 known to produce fertile eggs without copulation; in some of these the eggs 

 undoubtedly develop parthenogenetically, but the possibility remains that in 

 others the eggs may be fertilized by spermatozoa from the same worm. 



In many species of planarians reproduction occurs also by transverse fission, 

 usually in the post-pharyngeal region. In most of these species the worm 

 merely constricts and so divides itself into a head- and a tailpiece, each of 

 which heals the wound, forms the missing parts by cell division, localization, 

 and differentiation, and undergoes regulation of proportions. 



Regeneration. As may be demonstrated by a variety of experiments, many 

 species of planarians have great powers of regeneration (Fig. 11.8). The re- 

 generative process, by which even a small piece of the animal may form a per- 

 fect individual, involves healing of the wound surfaces and formation of a 

 small amount of new tissue at these regions, followed by the same processes 

 encountered in the growth of a new individual after fission. Totipotent or 

 formative cells are apparently instrumental in regeneration; these cells have 

 been interpreted as undiflferentiated cells of the mesenchyme which move into 

 an injured area and by repeated divisions produce the tissues required for 

 repair. In regenerative processes even small and irregular fragments exhibit 

 polarity, so that the axes of the original body are preserved in the new 

 individual. Each piece has the capacity to form a normal adult, except in 

 special cases in which an abnormal worm is formed — for example, an in- 

 dividual with two heads or two tails. 



Cellular Structure and Function. The epidermis is composed of cuboidal 

 or flattened cells, ciliated only in the ventral and lateral regions of the 

 body (Fig. 11.9). The epidermal cells rest upon a thick basement membrane 

 which functions as an elastic skeleton. Attached to the inner surface of this 

 membrane is a mesodermal musculature of longitudinal, transverse, and 

 diagonal fibers; dorsoventral fibers are also present, extending vertically be- 

 tween the dorsal and ventral basement membranes. The muscle fibers are 

 cytoplasmic extensions of cells whose nuclei are inconspicuous. As previously 



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