266 COMPARATIVE ANATOMY CHAP. 



nephridium which serves as uterus and oviduct. The intestinal canal is without 

 mouth or anus, a vascular system is wanting, nor have they the prostomium 

 which in the female is strongly developed as a proboscis. In short, apart from the 

 .sexual organs, they remain on the larval level. In some Myzostomidce there are so 

 called complementary males, which are considerably smaller than the hermaphrodite 

 individuals on which they live. Their organisation is like that of the hermaphro- 

 dites. As rudiments of ovaries and oviducts have been discovered in their bodies, 

 we can only consider these complementary males as originally hermaphrodites with 

 one-sided development of the male genital apparatus, or as young hermaphrodites 

 with the female genital apparatus not yet developed. 



XIII. Parthenogenesis. 



Reproduction by means of unfertilised eggs certainly takes place in the Rotatoria 

 and Bryozoa. It was formerly thought that only the summer eggs of the wheel 

 animalcules could develop without fertilisation, while the winter eggs must be fer- 

 tilised, but the act of fertilisation has not yet been observed in connection with these 

 latter. The statoblasts of the fresh -water Bryozoa are parthenogenetic eggs, and 

 such eggs are also found in some marine Bryozoa. These eggs are enclosed in hard 

 cases provided with many arrangements (air rings, processes, etc. ) serving to disperse 

 them in the air or water. 



XIV. Asexual Reproduction by Gemmation and Fission. 



Many worms, especially the Nemertina, Chcetopoda, Sipunculidce, Phoronis, and 

 the Bryozoa, are distinguished by a highly developed capacity of regeneration, which 

 is of the greatest use for the maintenance of the individual and of the race. Such 

 portions of the body as have been lost through adverse external circumstances, . 

 broken off, bitten off, etc., are quickly regenerated, even such as contain the most 

 important organs, e.g. the anterior part with the brain. Isolated broken off pieces 

 may occasionally be regenerated into whole animals. As already indicated, we may 

 perhaps some day be able to refer back the capacity shown throughout the animal 

 kingdom for asexual reproduction by gemmation and fission to such an accidental 

 multiplication by voluntary or enforced falling to pieces of the body with subsequent 

 regeneration. 



Among the Vermes this form of reproduction occurs in the Polychceta, Oligochceta, 

 and Bryozoa. 



Polychaeta. One of the Capitellidce, Clistomastus, constricts off (most probably 

 periodically) the posterior part of the body, which contains the sexual products, 

 reforming it again by regeneration. In a Syllis, Haplosyllis spongicola, which lurks 

 in holes at the bottom of the sea, the parapodia and setae of a number of the 

 posterior segments become more strongly developed as sexual maturity approaches. 

 The group of segments thus modified, containing the sexual products, severs itself, 

 and swims about freely in the sea as a sexual swimming bud, dispersing the sexual 

 products. In other Syllidce (Syllis, Autolytus] at the anterior end of the swimming 

 bud a new head is formed with highly developed eyes, and this takes place even 

 before it is severed. The detached swimming bud then represents a complete 

 individual (person), in which the sexual products ripen. The individual from which 

 the swimming bud has severed itself forms no sexual products, but is able at its 

 posterior end to produce new swimming buds. The detached swimming buds or 

 sexual animals may, apart from the fact that they contain the sexual organs, 

 be distinguished from the mother animal by other, chiefly external, points of 



