Hermaphroditism is characteristic of Chaetopoda Oligochaeta, Hirndinea, 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, and, 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, Archi-Annelida, the Gephyrean Sipnncididae and Echiuridae. They are conveyed away by nephridia, except 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 (1) 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 intestine (=archenteron), and a short rectum (=proctodaeum), which is terminal, 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.
1The early origin of genital organs as special cells has also been observed in Insecta, in Moina, among Cladocera (Crustacea), in JVematoda, and entoproctous Polyzoa. How far it may be significant 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 Poly'gordius, and the Serpulid Eupomatus; and a second, corresponding to the post-oral ring, has been detected in the
The body of the adult is formed by a lengthening and segmentation of the region between the post-oral ciliated ring and anus. The Trochosphere is probably derived from an organism (Trockozoon) resembling the early stage of a Turbellarian larva (Muller's larva), from which the Nemertean Pilidium is also undoubtedly derived 1. The prostomium of the Trochosphere must be taken to correspond with the aboral apex of the primarily ovate body of the Turbellarian larva, which undergoes a tilt in one direction, while the mouth, which is at first situate at the opposite end of the same vertical axis, comes to lie behind the apex; at the same time that side of the originally symmetrical body which is now opposed to the apex in its altered position commences to lengthen and grow into the body of the worm. The characteristic prae-oral ring of the Trochosphere is derived from the equatorial band of the simple larva. The body of the Vermian, like the Molluscan Trochosphere, grows in such a way that its posterior apex lies on the oral aspect of the equatorial ring, and the anus, which is never present in the Turbellarian is consequently, when formed, below, i.e. on the oral side of the ring.
On the contrary, in the Enteropneustan Tomaria and the larval form of all Echinoderms (except Crinoideaf), it is on the apical or aboral side of the equatorial ring, which in these instances takes a longitudinal direction.
The development of the Nemertean from the Pilidium, or from the larva of Desor, and of the Himdinea, is remarkable for the new formation of the permanent ectoderm and the discarding of the larval ectoderm, provisional nervous system, and musculature. The Hirudinea, like the Oligo-chaeta Chaetopoda, have no special larval form, a want probably due to an abbreviation of development. The Tomaria of some species of Balanoglossus has a resemblance to the Bipinnaria of Asteroidea. It differs from it as from the Holothurian and Echinoid larvae, and agrees with the Trochosphere in the possession of optic organs and a ganglionic rudiment at the apex of the first-named. See Hatschek on the head of Polygordius and the Trochosphere of Eupomatus, in Arb. Zool. Inst. Wien, vi. (i), 1885. Attention was first drawn to the prae-oral circular nerve by Kleinenberg. Its resemblance to the nerve following the edges of the ciliated lateral lobes in the Pilidium, as described by Salensky, is very striking, especially from the point of view of the possible origin of the Trochosphere from some Pilidium-like ancestor. See Salensky, Z. W. Z. xliii. 1886. It is generally held that the ventral nerve-cord originates independently of the supra-oesophageal ganglion.