ANNELIDS. 



433 



the segments of the body, and that the latter are defined externally by transverse 

 grooves, and internally by septa or partitions, which divide the body-cavity into a 

 series of compartments. 



Many-Bristled Group, — Order Polycilota. 



The Annelids of this order live exclusively in the sea, and may be referred to 

 two sections or suborders, the one being the wandering or roving species, which 

 have no fixed abode (Errantia), while the other sedentary forms live habitually in 

 tubes which they construct for the purpose. In the former of these two suborders 

 in accordance with the free 

 roving life, the front end of 

 the body is furnished with 

 a conspicuous lobe, or pro- 

 stomium, overhanging the 

 mouth, which bears eyes and 

 feelers. Those forms that 

 are carnivorous seize their 

 prey with sharp - hooked 

 teeth, visible at the end of 

 the proboscis. The struc- 

 ture of one of the parapodia 

 is shown in the accom- 

 panying figure. It consists 

 of two principal branches, 

 an upper (A) and a lower 

 (B), each being supported 

 by a long stout bristle (e, i). 

 The branches are further 

 divided into several well- 

 defined parts. For instance, 

 in the upper there is a con- 

 spicuous feeler or sensory 

 cirrus (a), and a bilobed 

 leaf-like plate (b, c), from 



parapodium and bristles of a polychjETOUS annelid, Heteronereis 

 cerstedi (much enlarged). 



the lower lobe of which 



(c) projects a cluster of arrow-shaped bristles. Analogous parts may be recognised 

 in the lower branch, / being the feelers, at the base of which is the leaf-like plate 

 (k); while the larger leaf -like plate (g) supports a second and larger tuft of 

 similarly-shaped bristles. A well-known example of this group is the sea-mouse 

 of the British shores (Aphrodite acwleata), a broad -bodied, somewhat slug-shaped 

 creature, commonly 3 or 4 inches in length. Like many of the species of marine 

 worms the sea-mouse is ornamented with iridescent hues, revealed by cleansing 

 the skin of the mud and sand with which it is usually coated. The back is 

 furnished with a double row of large overlapping scales, but in the British 

 species these scales are concealed by a close felt of hairs, although in another kind 

 vol. vi. — 28 



