72 THE EVOLUTION OF SEX. 



The genus Sagitta, which stands by itself, is hermaphrodite ; the same 

 condition is known as a rarity among the ancient brachiopods [Lingtcla), 

 but is frequent among the colonial Polyzoa. 



{4.) Ecliinoderniata. — The members of all the echinoderm class, except 

 one brittle star {Anipliiiira sqiianmta) and one genus of holothurians 

 {Synapla), have the sexes separate. 



(5.) Arthropods. — Among crustaceans, hermaphroditism is a rare ex- 

 ception, though it occurs in the majority of the fixed quiescent acorn-shells 

 and barnacles {Cirripedia). There it is associated with the presence of 

 small males, which Darwin called " complemental." In the Cymothoidre 

 (Isopods), we have a curious occurrence, somewhat like that oi Augiostoinuin 

 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 

 spare, 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 arach- 

 nids, otherwise unisexual, is found in the degenerate water-bears or sloth- 

 animalcules ( Tardigrada). 



(6.) Molluscs. — Most bivalves are of separate sexes, but exceptions 

 often occur — e.g.^ in common species of oyster, cockle, clam, &c. In the 

 case of the oyster, the familiar species {Ostrea ediilis) is hermaphrodite, 

 and a neighbouring species apparently unisexual. In both cases the organs 

 are the same, but in O. edulis the same intimate recesses of the reproduc- 

 tive 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 {Strcptoneiird) have the sexes 

 separate ; the members of the other series {Eiitliyiienra) are hermaphrodite. 



The sea-butterflies, or pteropods, are hermaphrodite, but the elephant's 

 tooth shells (vScaphopods) are unisexual. So in cuttle-fishes (Cephalo- 

 pods), the sexes are separate. 



§ 6. Degrees of Normal Herniaplwodiiisni. — From what has 

 been already said, it is evident that hermaphroditism may be 

 more or less intimate. As an entire plant, an Aniiti is herma- 

 phrodite, with female flowers on the better nourished lower 

 portion, and male flowers above. This may be paralleled by 

 the red coral, which is sometimes female as regards one branch, 

 and male as regards another. If we keep, however, to herma- 

 phrodite individuals, it is evident that an orchid, with stamens 

 and carpels united, is more closely hermaphrodite than a butter- 

 cup flower. So in a leech, with the ovaries far forward, and 

 independent of the long row of testes, the hermaphroditism is 

 less intimate than in a tunicate, where the testes and ovary may 

 form one mass, the male cells spreading over the surface of the 

 ovary. In the same way, the organ of a scallop, which exhibits 



