214 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



organs termed cirri (fig. 9). These out-growths, or para- 

 podia, are locomotor organs, and, by their active move- 

 ment in the living animal, enable it to creep or swim as 

 occasion demands. Each parapodium consists of a basal 

 piece from which extend outwards two almost equal, 

 usually bilobed, processes, a dorsal one (the notopodium), 

 and a ventral one (the neuropodium). In each of these 

 two rami of the parapodium there is a large setal sac, 

 formed by invagination of the epidermis, the lips of which 

 are well seen in Nereis. From the inner end or bottom 

 of this sac the setse arise, each being formed by products 

 from a single cell. The bundle of setse contained in each 

 sac may be protruded or retracted or moved in various 

 directions by means of a number of slender muscle bands 

 inserted into the bottom of the sac. In addition to these 

 setse, which may project a considerable distance beyond 

 the lips of the setal sac, there is in each notopodium and 

 neuropodium a stouter, dark-coloured, needle-shaped aci- 

 culum of a similar constitution to the setse. Its base 

 extends further inwards than the bases of the ordinary 

 setse, but its pointed tip projects only a short distance 

 beyond the surface of the parapodium. The two acicula 

 serve as an internal skeleton to the parapodium, and to 

 them the muscles which move the bundles of setse are 

 attached. On the dorsal side of the notopodium, and on 

 the ventral side of the neuropodium, is a finger-shaped, 

 or filamentous sensory process, or cirrus. 



The head or prostomium of Nereis (figs. 7 and 8) is 

 pre-oral. It is followed by the peristomium on the 

 antero- ventral surface of which the mouth is situated. 

 The prostomium bears on its dorsal surface two pairs of 

 eyes, iii front a pair of short sensory tentacles and ventro- 

 laterally a pair of much stouter sensory and muscular 

 palps. The peristomium, which bears some resemblance 



