cxxviii Introduction. 



' pseudhaemal vessels/ and a series of depuratory organs^ opening 

 internally into the perivisceral cavity, as well as externally on the 

 surface of the body. They are divided into two sub-classes, the 

 Chaetophora s. Chaetopodes, and the Discophora, accordingly as 

 their external locomotor organs are chitinous spines, or terminal 

 suckers. The Chaetophora again are divided into Polycliaeta s. 

 Branchiata and Oligochaeta, according to the number of their 

 locomotor and the presence of branchial appendages. The Disco- 

 phora, with the exception of Branchellion, have no external append- 

 ages locomotor, branchial, or tactile ; in the absence of appendicular 

 organs of the two latter kinds, as also in being hermaphrodite, 

 the Oligochaeta resemble the Discophora, and differ from the 

 Polychaeta. 



The Annulata may form tubes for the lodgment of their bodies 

 by the secretion either of calcareous matter [Serpula), or of organic 

 (chitinous?) substance (Onuphis), or by the agglutination of are- 

 naceous particles ( Terehella), and their integument, which is always 

 chitinogeuous, even in the Hirudineae, where it developes no ex- 

 ternal appendages, may develope chitinous appendages of the most 

 varied forms. But these chitinized outgrowths are with the single 

 exception of the operculum of Serpula, never indurated by any 

 deposit of calcareous salts, as is the case in Echiiiodermata and 

 Crustacea. The integument is frequently, even in adult Annulata, 

 found to be beset with cilia, especially in the anterior regions of 

 the body. This is the case with many Naidina, with Polyopli- 

 thalmus, with Siphonostomum gelatinosum, and with 8pio and 

 Tomopteris ; and in these two latter, as in many other genera, the 

 integument has been observed to contain tricho-cysts, or ' bacillar 

 corpuscles,' which have been compared with the acicular or urti- 

 cating organs of the Coelenterata, of certain Infusoria, of the 

 Apneustic Mollusca, of the Turbellarian Vermes, and, according 

 to M. A. de Quatrefages, of Synapta amongst the Echinoderms. 



All Annulata possess two layers of muscles in their body-walls — 

 an external consisting of circular, and an internal of longitudinal 

 fibres. To these an additional layer may be superadded internally, 

 and enter into the formation of the dissepiments which divide the 

 perivisceral cavity into its transverse and longitudinal compart- 

 ments. The digestive tract takes, with the exception of Chloraema, 

 a direct antero-posterior course, without describing any convolu- 



