The ventral nerve-cord is usually said to be derived from two epiblastic thickenings which unite at a later period with the cerebral ganglia, epiblastic thickenings of the prae-oral lobe; cf. note, p. 584. Two provisional renal organs open externally in front of the anterior- end of the nerve-cord in some Trochospheral larvae and larval Oligochaeta. They appear in the individual produced by fission of Aeolosoma (Vejdovsky). Their inner extremities end blindly1. Others may be present; see p. 582. The mesoblast forms two 'primitive' bands, at first continuous, afterwards segmented, of epiblastic or hypoblastic origin. The setae may appear before the parapodia; the noto- and neuro-podium may arise by the division of a simple parapodium, or the former appears before the latter. Some larvae possess bundles of long slender but provisional setae, comparable to the setae of some fossil forms (Agassiz)2.
Asexual reproduction by fission with or without gemmation occurs in some Oligochaeta (Aeolosoma, Chae togas tridae, Naidomorpha) and some Polychaeta (certain Serpulidae, and Syllidae). In Aeolosoma the worm increases in length (i. e. in number of somites); a somite near the centre of the body enlarges, forms a prostomium, and then separation takes place 3.
1 See Harmer, Q. J. M. xxv. 1885, P- 279; Vejdovsky, Oligochaeta, Prague, 1885, p. 162.
2 Claparede divided the larvae into Metachaetae with, and Perennichaetae without, these provisional setae.
3 The Oligochaete Lumbriculus variegatus appears to multiply simply by breaking in two, or, when alarmed or irritated, into several fragments ( = Schizogony). The parts or fragments develope new heads and new tails as may be necessary. See von Bulow, A. N. 49, pt. i. 1882; and for the histology of the regenerating tail, Id. Z. W. Z. xxxix. 1883. The Ctenodrihis monostylos of Zeppelin (Z. W. Z. xxxix. 1883) undergoes simple fission, and divides in the same way into fragments containing a few - in this case two or more - somites, which develope into a new worm. The Ct. partialis of von Kennel (Arb. Zool. Zoot. Inst. Wurzburg, v. 1882), which is, according to Vejdovsky, a Parthenope, multiplies by fission accompanied by gemmation. Both worms last named are marine; their hypodermis contains coloured gland cells similar to those of Aeolosoma; their nervous
The process is more complicated in Ckaetogastridae and Naidomorpha. A 'zone of fission' is formed between two somites as soon as the somites increase beyond a certain (? fixed) number. The 'zone' divides into an anterior and posterior portion, the latter forming a head for the posterior set of somites, the former giving rise to a series of new somites. Chains of zooids are thus produced, but the order of the zooids in a chain and the number that constitute a chain, appear to be inconstant (Semper). Under certain conditions, such as unfavourable climatic changes, the zooids become free, lengthen and develope sexual organs. In the Serpulids Protula and Filograna, the posterior somites are set free from the anterior with previous formation of a head. As to the Syllidae, the posterior somites in the genera, Syllis, Trypanosyllis, and Opisthosyllis develope sexual organs and bundles of long slender notopodial setae ( = Pubertats-borsten); a head is formed, and then the sexual somites, which are not constant in number, are set free as a separate worm1. The phenomena are more complicated in the Syllidian tribe Autolyteae. Chains of individuals are produced of which the last is the oldest.
The derived zooids develope, one by one, their sexual products and long notopodial setae, and are then detached. An anal somite is always formed by the zooid in front of the one upon the point of being detached. As the parent-form in these Syllidians remains non-sexual, an alternation of generations is set up. Syllis ramosa, which has been found inhabiting Hexactinellid sponges from the eastern seas is a more remarkable form still. It occurs in branched colonies. Small branches become sexual, develope a head with large eyes and long notopodial sexual setae, and are set free as males and females.
The fissiparous individual among the Oligochaeia above mentioned differs like an immature worm from the sexual individual by the absence of clitellum, genital setae, etc. as well as of sexual organs, and by a difference in the number of somites making up the body. The asexual Syllidian differs from the sexual by the absence of the long notopodial setae, and, where fission occurs, also in the shape of the head and size of the eyes. The sexual Heteronereis-form of certain species of Nereis differs from the immature individual by the shape and size of the para-podia of the genital somites which become adapted for swimming, by the presence of long slender setae, the conformation of the dorsal cirri especially, and the anal somite, and an increase in the size of the eyes. These changes are more marked in certain respects in the male, leading to a slight degree of sexual dimorphism. Such dimorphism, however, is consystem lies entirely in the hypodermis, and the sole pair of nephridia present appear to belong to the peristomium.
They are primitive forms, and are perhaps Oligochaeta. See the authors named; and for a discussion on the questions of affinity and generic names, Vejdovsky, System der Oligochaeten, 1885, PP- 164-66.
1 The non-sexual parent in Trypanosyllis Krohnii developes previously to separation a new peri-anal somite. Cf. Marion and Bobretzky. A. Sc. N. (6). ii. 1875, p. 36, spicuous in Autolytusprolifer (Syllidae), where the male has been described under the generic names, Polybostrichus, Diploceraea, and Crithida, and the female as Sacconereis. Polymorphic generations are found in Nereis Dutnerilii (Claparede). Certain individuals become sexually mature but are of separate sexes; others appear to become hermaphrodite ( = Nereis massiliensis); others again are metamorphosed into a Heteronereis, of which two forms are known, one large and tubicolous, the other small and pelagic.