iv VERMES GENITAL ORGANS 265 



In Dinophilus (Fig. 162, p. 246) the sexes are separate. An 

 ovary lies on the outer surface of the intestinal wall, ventrally 

 on the boundary between the mid- gut and the hind-gut. It may 

 perhaps be developed out of an endothelium, which, however, has 

 not yet been proved to exist. The ripe eggs fall into the body 

 cavity and are emptied out through a pore which forms temporarily 

 in the body wall in front of the anus. The male sexual organs are 

 not yet sufficiently known. 



In the male of D. apatris, at the posterior end, a conical organ 

 (penis) is found, which lies in a sac out of which it can be protruded 

 and into which it can be withdrawn. 



The Chcetognatha (Fig. 152, p. 227) are hermaphrodite. The 

 ovaries are two long tubes which lie in the posterior part of the 

 two lateral chambers of the trunk cavity. Throughout their whole 

 length they are attached laterally by a mesentery to the body wall, 

 this mesentery enclosing them all round like a sac. On the outer side 

 of each ovary, and enclosed in its mesentery, lies a long oviduct, 

 ending blindly to the front, and opening outwardly behind near the 

 septum which separates the trunk and the tail cavities. It is not yet 

 known how the eggs reach the oviduct and are discharged. The two 

 testes lie as lateral cell thickenings or germ masses projecting into the 

 body cavity from the body wall of the tail cavity. Groups of 

 young sperm formative cells sever themselves and fall into the two 

 lateral chambers of the tail cavity. The ripe spermatozoa are dis- 

 charged through the sperm duets. In each sperm duct we can dis- 

 tinguish an inner ciliated funnel, a duct, and a vesicle ; these open 

 externally at the sides of the caudal segment. Thus in the arrange- 

 ment of the reproductive organs, especially in that of the male genital 

 apparatus, we find great similarity between these animals and the 

 Chcetopoda, Sipunculidce, Phoronis, and the Bracliiopoda. 



Sexual Dimorphism in Worms. Apart from the difference between the sexual 

 organs and outer copulatory organs, there are, in many worms in which the sexes 

 are separate, insignificant external differences between the male and the female. 

 In some worms, however, the differences in inner organisation cause a remarkable 

 sexual dimorphism in outer appearance and in size between the two. This is especi- 

 ally the case in the Rotatoria, Dinophilus, and Bonellia. It is always the male 

 which, in comparison with, the female, appears reduced and generally dwarfed. The- 

 Rotatorian male is smaller than the female, with degenerated enteric canal and 

 simplified wheel apparatus. The males are known only in the minority of genera 

 and species, and are much rarer than the females. In Seison alone the males and 

 females are alike. The fertilisation of the Rotatorian eggs has till now not been 

 observed. The females usually produce two sorts of eggs delicate-skinned summer 

 eggs, and hard -shelled eggs which last through the winter. In Dinophilus 

 vorticoides the two sexes are not different ; in D. apatris the male is smaller, with 

 ciliated rings. Mouth, intestine, and anus are wanting. The males of Bonellia,. 

 which are ciliated all over, and are in appearance not unlike the Rhabdocaelan 

 Turbellaria, are minute in comparison with the females ; these males live parasitic - 

 ally on the female in varying numbers on the proboscis, in the oesophagus, or in the 



