2/0 



MANUAL OF ZOOLOGY. 



bands, one about each extremity, or a series of bands (fig. 131, 

 B and C). The head, with its feelers and eye-specks, appears 

 at one extremity, whilst the segments of the body begin to be 

 formed at the other. Each segment is developed in four parts, 

 the two principal ones forming half-rings, united by shorter 

 side-pieces, from which the setigerous foot-tubercles are de- 

 veloped. The ciliated band or bands finally disappear, and 

 new rings are rapidly added by intercalation between the head 

 and the segments already formed. 



Amongst the best known of the Errantia is the common Lob-worm 

 (Arenicola piscatorum, fig. 133, C), which is used by fishermen for bait. 

 The Lob-worm lives in deep canals, which it hollows out in the sand of 

 the sea-shore, literally eating its way as it proceeds, and passing the sand 





Fig. 133. Errant Annelidas. A, Hairy-bait (Nepkthys) ', B, Sea-mouse (Aphrodite) ', 

 C, Lob-worm (Arenicola). (After Gosse.) 



through the alimentary canal, so as to extract from it any nutriment which 

 it may contain. It possesses a large head, without eyes or jaws, and with 

 a short proboscis. There are thirteen pairs of branchiae placed on each 

 side in the middle of the body. 



In the Sea-mouse (Aphrodite, fig. 133, B), the back is covered with a 

 double row of membranous imbricated plates, which are called "elytra," 

 or ' ' squamae, " and respiration is effected by the periodical elevation and 

 depression of these plates, whereby water is alternately admitted into, and 

 expelled from, a space beneath them. This space is separated by a mem- 

 brane from the perivisceral cavity below, and contains the gills in the form 

 of small fleshy crests. The pharynx is thick and muscular, and can be 

 everted like a proboscis, and the intestine has a number of lateral branched 

 caeca. 



In the Nereida, or "Sea-centipedes," the body is greatly elongated, and 

 consists of a great number of similar segments, with rudimentary branchiae. 



