594 THE ANIMAL KINGDOM. 



either consists of a uniform series of somites, as in Oligochaeta and Poly- 

 chaeta Errantia, or it may be divided into an anterior region, the thorax, 

 and a posterior, the abdomen, with differing somites as in Polychaeta 

 Tubicola. The number of divisions may be greater, as in Chaetopteridae. 

 The terminal, anal, or pygidial somite is often much reduced. A post-anal 

 cirriform part of the body appears to be present in Nephthys (Pruvot), and 

 in the Oligochaete Criodrilus there are seven abbreviated post-anal somites 

 (Vejdovsky). 



The locomotor setae are implanted in Polychaeta in parapodia (infra). 

 They are grouped, as a rule in large numbers, into bundles which are 

 either single or double according as the parapodium is uni- or bi-ramose. 

 The number of setae in a bundle, their size and shape is often characteristic. 

 A stout seta or aciculum very commonly forms the centre of the bundle. 

 Aphrodite among Polychaeta is remarkable for having large numbers of 

 hair-like setae growing from the notopodium. Some of these are iridescent, 

 others form a felt-work over the dorsum. A similar but more sparingly 

 developed felt-work is occasionally present in the allied genus Hermione. 

 The setae of Oligochaeta are grouped in aquatic and some terrestrial forms 

 in a dorsal and a ventral set. In other terrestrial forms there may be four 

 setae, implanted singly on each side of a somite or a complete or in- 

 complete ring of single setae girthing it. The number of setae in a bundle 

 is always small, and in some terrestrial Oligochaeta (Lumbricus,Anteus, &c.) 

 is reduced to two. The dorsal setae of the aquatic Oligochaeta are often 

 remarkably long. The setae are chitinoid, their shape and size vary re- 

 markably and are adapted frequently to the necessities of the worm, 

 e. g. the hooked ventral setae of many Tubicola, by means of which they 

 creep along their tube. They are moved by special muscles, protrusor as 

 well as retractor, and originate from cellular sacs or trichophores, invagi- 

 nations of the hypodermis. Each seta in a bundle is the product of a 

 single cell, and either springs from a separate sac or together with others 

 from a common sac. They are rarely entirely absent, as e. g. from the 

 parapodia of Tomopteris. In the Oligochaete Anachaeta the sacs persist, 

 but do not develope setae. 



The parapodia of Polychaeta are hollow lateral elevations of the body- 

 wall, either simple (uniramose) 1 or divided (biramose) into a dorsal noto- 



cerebral ganglia indicate the presence of somites is perhaps scarcely tenable ; at least, it is quite 

 as likely that its division into lobes is the consequence of an increase in the number of cephalic 

 appendages. 



The first ventral or sub-oesophageal ganglion is contained in the peristomial somite. It may 

 give off one, or two {Eunice), or three (Phyllodoce) pairs of nerves indicating a fusion of somites. 

 Actual fusion of a setigerous somite in the young animal with the true peristome has been observed, 

 e.g. in Nereids ; cf. Langerhans, Z. W. Z. xxxii. 1879, p. 517. The appendages of the peristome 

 may be parapodia with setae, or cirri usually much lengthened, and then termed by Claparede 

 tentacular cirri. 



1 A few setae are found at the base of the dorsal cirrus in Eunicidae. Tufts of fine setae 



