CHAETOPODA. 593 



CLASS CHAETOPODA. 



Mtdtisegmental Vermes, with a more or less prominent prostomial lobe ; 

 tvith locomotor organs in the shape of chitinoid setae, implanted either in the 

 body -wall or in special elevations, the parapodia ; with a body divided into 

 somites by external f^lrrows and typically by internal fibro-muscular septa. 

 Cilia are restricted to special regions or certain organs. The nervous system 

 consists of a pair of cerebral or prostomial ganglia, and a paired ventral cord 

 ^vith distinct or indistinct ganglia. The nephridia are typically repeated in 

 each body-somite ; the genital organs may be similarly repeated or be restricted 

 to certain somites, usually the anterior. Development is direct or with a 

 metamorphosis, 



Of the two orders, Polychaeta and Oligochaeta, into which the class 

 is divisible, the former is usually distinguished by the presence of para- 

 podia and by appendages to the head and somites, viz. antennae, cirri, and 

 branchiae. 



The division of the body into somites is not well-marked in some 

 Polychaeta Ttibicola. The number of somites present varies much, and may 

 attain to several hundreds, as in some Polychaeta Errantia and terrestrial 

 Oligochaeta. Annulation of the somites, as in Hirudinea, is very rare. The 

 head is generally distinct, and consists of a prostomium and peristomium. 

 The former varies in size, and in some Tubicola and terrestrial Oligochaeta 

 becomes obsolete 1 . It usually bears in Polychaeta a certain number of 

 appendages, the antennae or tentacles. Two which originate . from its 

 inferior aspect, and differ structurally as well as by their innervation from 

 the other appendages, are often termed palpi. The numerous extensile 

 tentacles of the Terebellidae, which contain a prolongation of the coelome, 

 and the gill-processes so-called of the Serpulidae, appear to belong to the 

 prostomium, as they originate in front of the ring of cilia which limits 

 posteriorly the prae-oral lobe of the larva. The peristomium or buccal 

 somite is sometimes very similar to the following somites, sometimes 

 markedly different, and it may coalesce with 1-3 of them 2 . The body 



1 The prostomium of the adult Chaetopod is certainly a rudiment of the larval prae-oral lobe, 

 when it contains the supra-oesophageal ganglion. It is, however, in some cases, e. g. some 

 Oligochaeta, an outgrowth of the following somite, if Vejdovsky's statements are right. Cp. 

 p. 197. 



2 The number of prostomial antennae varies from two to five. When there are five there is a 

 median azygos antenna, a pair of antero- or supero- lateral antennae, and a pair of postero- or infero- 

 lateral ; all alike innervated from the cerebral ganglia. The latter are divisible, according to Pruvot 

 (A. Z. Expt. (2), Hi. 1885), into an antennary and a st ornate-gastric portion. The former supplies the 

 antennae, and where there are five of these structures, as in Eunice, it is subdivisible into a part 

 supplying the median and antero-lateral antennae, and a second part supplying the postero-lateral 

 pair. The stomato-gastric portion innervates the palpi, and gives origin to the stomato-gastric nerves 

 as well. The homology of the Serpulidan branchiae with palpi is proved by their innervation from 

 the stomato-gastric portion of the cerebral ganglia. Pruvot's theory that the subdivisions of the 



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