606 THE ANIMAL KINGDOM. 



Polytrochae with bands appearing on the somites between the two telo- 

 trochous bands and subdivisible into (i) Polytrochae proper with complete 

 bands ; (2) Nototrochae with only dorsal half-bands ; (3) G aster otrochae 

 with only ventral half-bands ; and (4) Amphitrochae with both dorsal and 

 ventral half-bands which do not correspond ; Mesotrochae with one or two 

 median bands but devoid of the two telotrochous bands (the Chaetopteridae 

 alone). A flagellum or a bunch of cilia are often situated at the apex of 

 the prae-oral lobe, and there may be an anal patch of cilia. The prae-oral 

 band is frequently double, and the mouth is then situated ventrally between 

 the two. A stomodaeal and proctodaeal invagination probably always 

 occur, at least during growth. The blastopore is said to persist as the 

 primitive oral opening in some Oligochaeta. It closes in Serpula in what 

 corresponds to the median ventral line, and the mouth and anus are 

 formed in positions coinciding severally to its two extremities. The 

 archenteron is more or less saccular, and the anus posterior. The larval 

 digestive tract makes a curve concave ventrally. 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. 5 84. 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 blindly 1 . 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, Chaetogastridae, Naidomorphd) 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- 2 79 5 Vejdovsky, Oligochaeta, Prague, 1885, p. 162. 



2 Claparede divided the larvae into Metachaetae with, and Perennichaetae without, these 

 provisional setae. 



3 The Oligochaete Lttmbriculus variegattis 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 Billow, A. N. 49, pt. i. 1882 ; and for the 

 histology of the regenerating tail, Id. Z. W. Z. xxxix. 1883. The Ctmodrilus monostylos of Zeppelin 

 (Z. W. Z. xxxix. 1883) undergoes simple fission, and divides in the same way into fragments con- 

 taining a few in this case two or more somites, which develope into a new worm. The Ct. 

 fardalis 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 



