226 CASES SIMULATING ALTERNATION OF GENERATIONS. 



a simpler structure are met with in some Insects, Crus- 

 taceans, and other Invertebrate animals.* 



Much in the same way the eggs of some Insects, Anne- 

 lidans, and Molluscs,"!* after acquiring the usual character of 

 impregnated ova, become enclosed in peculiar envelopes ; 

 and some of these structures also are said, though on rather 

 doubtful evidence, to have certain motile powers. 



In the simple fact of the enclosure of the reproductive 

 bodies in appropriate receptacles there is nothing very pe- 

 culiar ; it is merely, so to speak, the application to masses 

 of spermatozoa and ova, of what we are familiar with in the 

 case of single ova, in the formation of the shell of the bird's 

 egg by the lower portion of the oviduct, or of the decidua of 

 the mammalian foetus by the uterus. It is more difficult to 

 decide on the proper relations of the receptacles in those 

 cases in which they exhibit spontaneous motions, or other 

 indications of independent vitality. They may perhaps be 

 considered in the light of special organs of the parent's body, 

 destined to receive the corpuscules, and to be afterwards 

 thrown off for their protection and dispersion, like the 

 Hectocotylus-arm of the Cuttlefish, just referred to. 



But such egg-sacs, or spermatophores, though in a certain 

 sense distinct forms in the genetic cycle, intermediate between 



* Siebold's Compar. Anatomy, I. 290-348. Annals of Nat. Hist., 

 2d Ser., XVI., 150, XIX., 165. In some Crustacea the spermatophore 

 seems to be formed by the coagulation of the outer film of the seminal 

 fluid, on coming in contact with sea- water, the shape depending on that 

 of a groove in the shell in which the spermatophore is modelled, as it 

 were. Coste Annals 3d Ser., II., 197. In Geophilus a genus of 

 Myriapoda the impregnation seems to be effected by the female 

 coming in contact with the spermatophore, which is suspended in the 

 centre of a net stretched across one of the subterranean galleries of 

 the insect. Annals, XIX., 165, 2d Ser. 



f Blatta among Insects, Purpura and other genera of the Pectini- 

 branchiate Mollusca, Nemertini, Hirudinei, and Lumbricini, among 

 Annelida. The egg-sac of the earthworm (its so-called egg) contains two 

 embryos that of the leech as many as eighteen. 



