VERMES. 583 



Hermaphroditism is characteristic of Chaetopoda Oligochaeta, Hirudinea, 

 Trematoda, Cestoda, Turbellaria, and Chaetognatha. It generally takes the 

 form of successive hermaphroditism, i.e. one of the two genital products, 

 nearly always the male, ripens at an earlier period than the other. Self- 

 impregnation takes place sometimes, and probably often, in Trematoda,zx\d, 

 so far as is known, it is the rule in Cestoda. Reciprocal impregnation is the 

 rule in other instances. The Nematode genus Angiostomum is a unique 

 example of an organism which, though anatomically speaking a female, is 

 a self-impregnating hermaphrodite. Both genital products are formed 

 from coelomic epithelium in Chaetopoda, Arc hi- Annelida, the Gephyrean 

 Sipunculidae and Echiuridae. They are conveyed away by nephridia, ex- 

 cept perhaps in the Chaetopoda Oligochaeta (see pp. 207-8). In Hirudinea 

 as in Chaetognatha, the genital glands are derived from special cells set 

 apart at a very early period 1 . The ducts of the ovaries and testes are 

 developed independently of the glands in the first-named ; and Nusbaum 

 holds that they are two pairs of modified nephridia. In Chaetognatha, 

 whilst the ovaries acquire ducts, but how is unknown, the testicular 

 products are thrown into the coelome and carried away by open canals. 

 In other classes of Vermes, and in Priapulidae among Gephyrea, there are 

 genital organs either continuous or becoming continuous with ducts, or else 

 simply bursting externally as in Enteropneusta and Nemertea. Accessory 

 organs to the genitalia are present in the hermaphrodite groups, and are in 

 some instances extremely complicated. 



Peculiarities of development are noted under each class. There is, 

 however, a larva, the Trochosphere or Trochophora, which occurs in Poly- 

 gordius in a simple and characteristic form : in Chaetopoda Polychaeta, and 

 in Echiurus among Gephyrea. The general characters of this larva are (i) 

 the presence of a prae-oral lobe with apical nerve ganglion, the future 

 supra-oesophageal ganglion or thickening, from which a nerve, the future 

 oesophageal commissure, runs backwards towards the anus (? in all) ; (2) of 

 a prae-oral ring of cilia to which are often added a post-oral ring and an 

 adoral band between the two rings ; (3) of an archicoele ; (4) of a ventral 

 mouth leading into an oesophagus ( = stomodaeum), a stomach, and in- 

 testine (=archenteron), and a short rectum ( = proctodaeum), which is ter- 

 minal, and sometimes surrounded by a patch or ring of cilia. There are 

 optic organs on the ganglionic thickening, and contractile cords connecting 

 it to the oesophagus, as well as provisional cephalic renal organs (supra] 2 . 



1 The early origin of genital organs as special cells has also been observed in Insecta, in Moina, 

 among Cladocera {Crustacea), in Nematoda, and enloproctous Polyzoa. How far it may be sig- 

 nificant it is difficult to say. The ducts in Insecta are in most instances partly derived from the 

 genital cells, partly from invaginations of hypodermis, the latter sometimes wanting. The same 

 is true as to Nematoda. 



2 A circular nerve corresponds to the prae-oral ring in the Trochosphere of Polygordius, and the 

 Serpulid Eiipomatus ; and a second, corresponding to the post-oral ring, has been detected in the 



