124 



INVERTEBRATE ANIMALS. 



The most familiar of the Tubicola is the Serpula (Fig. 

 48, #), the contorted and winding tubes of which must be 

 known to every one as occurring on shells or stones on the 

 sea-shore. One of the cephalic filaments in Serpula is much 

 developed, and its extremity forms a kind of conical plug 

 which serves to close the mouth of the tube when the animal 

 is retracted within it. In Spirorbis (Fig. 48, b) the shelly tube 

 is coiled into a flat spiral, which is fixed to some solid object. 

 It is of extremely common occurrence on the fronds of sea- 

 weed, and on other submarine objects. 



ORDER IV. ERRANTIA. The Annelides comprised in this 

 order are called " errant " (Lat. erro, I wander), or " roving," 

 from the fact that they all lead a free existence, and are never 

 confined in tubes. They have always lateral unjointed ap- 

 pendages, or foot-tubercles (Fig. 49), which carry tufts of 



FIG. 49. Errant Annelides. A. Hairy Bait (yepJithys); B. Sea-Mouse (Aphrodite) ; C. 

 Lobworm (Arenicola). (After Gosse.) 



bristles and a soft, jointed filament. The anterior rings of 

 the body are usually so modified as to form a sort of head, 

 which is provided with eyes and with two or more feelers, 

 which differ from the antennas of insects and Crustaceans in 

 not being jointed. The mouth is placed on the inferior sur- 

 face of the head, and is sometimes furnished with one or more 

 pairs of horny jaws, which work from side to side. The upper 



