68 THE EVOLUTION OF SEX. 



thus physiologically hermaphrodite, and at the same time self-impreg- 

 nating. Approaching the higher annelid worms, we find the prim- 

 itive Protodrilus hermaphrodite; the earthworms are constantly so, but 

 all their marine relatives have the sexes separate. The genus Sagitta, 

 which stands by itself, is hermaphrodite; the same condition is known 

 as a rarity among the ancient brachiopods {Linguld), but is frequent 

 among the colonial Polyzoa. 



4. Echinodermata. — The members of all the echinoderm class, 

 except one brittle star (Amphiiira squamata) and one genus of holo- 

 thurians (Syiutptd), have the sexes separate. 



5. Arthropods. — Among crustaceans, hermaphroditism is a rare 

 exception, though it occurs in the majority of the fixed quiescent 

 acorn-shells and barnacles (Cirripedid). There it is associated with 

 the presence of small males, which Darwin called ' ' complemental. ' ' 

 In the Cymothoidcs (fscpods), we have a curious occurrence, some- 

 what like that of Angiostomum above noticed. ' ' The sexual organ 

 of the young animal is male, of the old, female in function." In 

 such cases, one must remember the antithesis between the body 

 proper and the reproductive cells. In youth the demands of the 

 body during growth are greater; there is no anabolic surplus to 

 gpare, all goes to increase the body. When mature size is reached, 

 and both growth and activities lessened, there is more likelihood 

 of anabolic preponderance in the reproductive, as opposed to the 

 vegetative, system. 



Myriopods and insects have always separate sexes, excluding of 

 course abnormal hermaphroditism among the latter. An exception 

 among arachnids, otherwise unisexual, is found in the degenerate 

 water-bears or sloth-animalcules (Tardigradd) . 



6. Mollusks. — Most bivalves are of separate sexes, but exceptions 

 often occur, — for example, in common species of oyster, cockle, 

 clam, &c. In the case of the oyster, the familiar species Ostrea edulis 

 is hermaphrodite, and a neighboring species apparently unisexual. In 

 both cases the organs are the same, but in O. edulis the same inti- 

 mate recesses of the reproductive organ produce at one time ova, at 

 another time sperms. 



The snails, or gasteropods, are divided into two great groups, 

 according to the twisting of their nerves. The one group {Strepto- 

 neurd) have the sexes separate; the members of the other series 

 {EuthyneurcL) are hermaphrodite. 



The sea-butterflies, or pteropods, are hermaphrodite, but the ele- 

 phant' s-tooth shells {Scaphopods) are unisexual. So in cuttlefishes 

 (Cephalopods) the sexes are separate. 



