CHAETOPODA. 609 



Silurian, Devonian and Carboniferous have been identified as Eunicidan 

 jaws. 



The class Chaetopoda contains, in addition to the two orders Polychaeta and 

 Oligochaeta, two others, the Chaetopoda ectoparasitica and the Archi-Chaetopoda, 

 described separately here, because of their interesting peculiarities. The first- 

 named order contains highly modified worms, but its position among Chaetopoda 

 must be regarded as settled by the discovery of the larval form. The Archi- 

 Chaetopoda resemble the Archi- Annelida (infra, p. 613) in the large size of the peri- 

 stomial somite, the similarity inter se of the remaining somites, and in the position 

 of the nervous system within the hypodermis, as well as in the connection of the 

 intra-muscular nervous plexus with cells of the hypodermis. 



Chaetopoda ectoparasitica = Myzostomidae. There are two genera included in 

 this group Myzostoma, parasitic on various Comatulidae and a few Pentacrinoids, 

 and Stelechopus, parasitic Qn.fyocrinus sp. ? The body oi Myzostoma is symmetrical 

 and non-segmented, usually disc-shaped, occasionally elongate. It is provided with 

 five pairs of short ventral and radially arranged parapodia, each armed with a 

 chitinoid hook and a supporting rod j five pairs of suckers commonly alternate in 

 position with the parapodia. The margin of the body has ten pairs of cirri at least, 

 or a larger number, which are furnished with terminal bundles of stiff sensory (?) 

 setae and a ventral furrow lined by adhesive cells. There is a chitinoid cuticle, and 

 cilia occur in isolated patches on the dorsal and ventral surfaces. The nervous 

 system consists of a large ventral ganglion, with five pairs of lateral nerves. A 

 supra-oesophageal ganglion appears to be absent. The alimentary canal consists of 

 a protrusible muscular proboscis, an oesophagus, a stomach, which gives off lateral 

 branched caeca, and a short rectum. Mouth and anus are generally on the ventral 

 surface, the former anterior, the latter posterior in position. There are no nephridia. 

 The majority are hermaphrodite.. The testes are ventral, paired, and branched 

 except in the dioecious forms, and the male apertures are lateral, and placed 

 externally to the third pair of parapodia. The ovaries are represented by a 

 number of caeca on the dorsal surface ; an oviducal tube opens into the dorsal 

 aspect of the rectum. M. cysticolum has rudimentary testes; M. tenuispimtm, 

 inflator, Murrayi, are dioecious, i.e. either male or female. M. glabrum, though 

 hermaphrodite, has small ' complemental ' males, like certain hermaphrodite Cirri- 

 pedia (p. 537). M. glabrum and cirriferum have larvae provided with a post-oral and 

 a prae-anal band of cilia, with a larval supra-oesophageal ganglion, and two bundles 

 of provisional setae ; they closely resemble the larval Nereis Dumerilii. The position 

 of the Myzostomidae in the class Chaetopoda must accordingly be regarded as a 

 settled fact. 



Stelechopus differs from Myzostoma in being elongate, in the parapodia not 

 being radially arranged, in the absence of suckers and cirri, and in the absence of 

 lateral caeca to its alimentary canal ; ? hermaphrodite. 



Von Graff, Challenger Reports, x. 1884; Id. Das Genus Myzostoma, Leipzig, 

 1877. Suckers, Niemiec, Recueil Zool. Suisse, ii. 1885. Development, Beard, 

 Mitth. Zool. Stat. Naples, v. 1884. 



Archi-Chaetopoda Saccocirridae. This order is represented by the marine 

 Saccocirrus papillocercus from the bays of Sebastopol and Marseilles. It has a 



R r 



