CHAPTER VIII. 



THE BIOLOGY OF AN ANIMAL (Continue^. 

 THE EARTHWORM. 



Reproduction. The general nature of the reproductive func- 

 tion has already been fully explained (p. 88) and illustrated in 

 the case of the fern. Unlike the fern, the earthworm multiplies 

 only by sexual reproduction or gamogenesis. This animal must 

 therefore be compared as to its reproduction with the oophore, 

 not with the sporophore, of Pteris, and there is no alternation of 

 generations (cf. p. 89). 



Germ-cells. Here, as in the fern, male and female cells are 

 borne by the same individual, so that the earthworm is bisexual 

 or hermaphrodite. The male and female germ-cells are very 

 similar to those of the fern, the former (spermatozoa) being very 

 minute and actively mobile, while the latter (ova or ergs) are 

 much larger and immobile. The ova are produced in special or- 

 gans, the ovaries, and the spermatozoa in organs known as sper- 

 maries or testes. 



The ripe ovum (Fig. 83, ov) is a nearly typical cell, agreeing 

 closely with that of the star-fish (Fig. 20), except that it has no 

 membrane. It is still customary to apply to ova the old termi- 

 nology, calling the cell-substance vitettus, the membrane mtelline 

 membrane, the nucleus germinal vesicle, and the nucleolus ger- 

 minal spot. 



The ripe spermatozoon (Fig. 83, sp) is an extremely minute 

 elongated cell or filament tapering towards one end (tail) to a 

 point, and having a slight enlargement at the other, the head. 

 The latter is really the nucleus of the cell enveloped by a thin 

 layer of protoplasm. The tail of an active spermatozoon is 

 virtually a long cilium (p. 31), which by vigorous lashing drives 

 the whole cell along head-foremost, very much as a tadpole is 

 driven by its tail. 



Since the ovaries and spermaries give rise to the germ-cells, 

 they are called the essential organs of reproduction. Besides 



