66 THE EVOLUTION OF SEX. 



vertebrates are either casual (par. Ill) or partial (par. IV). Among 

 invertebrates, true hermaphroditism is, however, of frequent normal 

 occurrence; for example, in sponges, ccelenterates, worm-types, and 

 mollusks. It is necessary to take a brief survey of some of these. 



i. Sponges. — As already mentioned, the sex-cells of sponges 

 start simply among the other components of the middle layer (jneso- 

 gloea) of the body. It is at least possible that in any sponge they 

 may develop either into ova or into sperms, or into both, within the 

 same organism, according to nutritive and other conditions. The 

 facts, however, are these. Many sponges are only known in a uni- 

 sexual state, while others are genuinely hermaphrodite. But among 

 the latter it is not uncommon to find (for example, in Sycandra 

 rapha?ius) that the production of one set of elements preponderates 

 over the other, and thus we have hermaphrodites with a distinctly 

 male or female bias. In other words, they are verging toward unisex- 

 uality. It does happen in fact (for example, in Oscarella lobularis) that 

 a species normally hermaphrodite may exhibit unisexual forms. It is 

 possible, of course, in such cases one set of sexual elements may have 

 been wholly discharged, or may even have been overlooked in obser- 

 vation; but there is no improbability against the supposition that a 

 preponderance of favorable nutritive conditions might induce a form 

 normally hermaphrodite to become wholly female. This, as we have 

 seen above, is what some believe to take place in the individual history 

 of higher forms. 



2. Ccelenterates. — The members of this class are higher, in having 

 the production of the sex-cells more restricted to definite regions, 

 tissues, organs, or even "persons." The highly active Ctenophores, 

 like Berbe, are all hermaphrodite, and that very closely. On one 

 side of the meridional branches of the alimentary canal ova arise, on 

 the other side spermatozoa. Among sea-anemones and corals the 

 hermaphrodite condition appears in a number of cases, but is some- 

 times obscured, corresponding to different physiological rhythms in 

 the life of the organism. The genus Cora/Hum (the red coral of com- 

 merce) is peculiarly instructive. The whole colony may be unisexual, 

 or one branch of the colony, or only certain individuals on a branch, 

 while genuine hermaphroditism of individual polyps also occurs. 

 Among hydrozoa (zoophytes, swimming-bells, jellyfish), hermaphro- 

 ditism is a rare exception, or, we may almost say reversion. The 

 common hydra, which is a somewhat degenerate type, is hermaph- 

 rodite, though at the same time individuals may be found with 

 only ovary or only testes. Elentheria is also hermaphrodite, and 

 " abortive ova occur in the male of Gonothyrea loveni." Sometimes 

 a colony is hermaphrodite ^Dicoryne) , but the stems and individuals 



