1867.] The Systematic Study of Annelids. 353 



that sort of foot which is divisible into two branches, a noto- 

 podiuni and a neuropodium (Figs. 7 and 8, nte. nrc.), each of these 

 two parts carrying a cylindrical appendage or cirrus and its bunch of 

 setae. The notopodial cirri in the two figures differ considerably, 

 resembling the peristomial cirri : they are smooth in Lepidonotus, 

 but carry short papillose hairs in Harmothoe. The group to 

 which the Polynoina belong is remarkable for having flat scale-like 

 bodies covering the back, placed in pairs on the alternate rings, or, 

 as in the Sigalionina, on nearly every ring. The scales are in 

 many species very beautiful objects, and furnish very important 

 means of distinguishing forms. In the genus Lepidonotus there 

 are only twelve pairs, and these adhere firmly to their attachment ; 

 in Harmothoe there are fifteen, or sometimes, twenty pairs, which 

 very readily slip off from the body. In Figs. 3, 4, the scales of 

 the forms we are noticing are drawn. Those of H. imbricata ex- 

 hibit the greatest variation in colour, being brown, black, purple, 

 yellow, or mottled with these colours. 



Let us glance now at another very different-looking group of 

 Annelids. The Nereids are long snake-like worms, often attaining 

 a length of ten or twelve inches. In the plate, the head of Nereis 

 pelagica and some of its setae and a foot are drawn (Figs. 11, 12, 13), 

 the parts homologous with those of the Polynoina just described 

 are similarly lettered. The very characteristic form of the setae 

 is of great value in grouping the species of this family as well as 

 the lobation of the feet. 



The suppression or modification of the parts of the foot and their 

 homologues attached to the head, constitute the essential differences of 

 the various tribes of marine worms. In some, the rings immediately 

 succeeding the head differ from the most posterior in the form and 

 character of their appendages; in most of these the appendages of the 

 head are curiously modified so as to form long filamentous branchiae, 

 Fig. 15, and sometimes also in tubicolous species an " operculum " 

 which closes the tube in which they live. The greater or less develop- 

 ment of a thoracic region has thus led to the division of marine 

 Chaetopods into two principal groups, the Errant and the Sedentary, 

 the latter group comprising those in which the thoracic region is 

 present ; a third group is sometimes distinguished which bear a 

 superficial resemblance to the earth-worms, and have been, by some 

 writers, associated with the Sedentary, by others with the Errant 

 forms. The modifications of the foot and its appendages are the most 

 remarkable in the Errant group, whilst the head exhibits the 

 greatest peculiarities in the Sedentariae. In Phyllodoce (an errant 

 genus) the notopodial cirrus has the form of a large leaf-shaped 

 appendage; the rings forming 'the animal being very numerous, 

 there are often more than a hundred pairs of the appendages, fre- 

 quently brilliantly coloured, with which the animal rows itself 



