308 PECTINABIA. ORDER TERRICOL.E. 



and they carry these about with them, when roaming in search of 

 food. It is perhaps in consequence of their locomotive powers, 

 which give them a greater facility of selection, that they construct 

 their shelly tubes with so much more regularity than other 

 Annelidans. These tubes, usually about two inches long, may 

 be frequently picked up on our shores. Some of this group, 

 however, are fixed like the other Tubicolas, and form their tubes 

 less regularly. Their bodies are doubled up, as it were, within 

 these envelopes ; so that the intestine terminates in a tube which 

 is curved back over the head. 



ORDER III. TERRICOLJE. 



842. The Annelidas of this Order have a cylindrical body, 

 tapering to a point at its extremities, and furnished only with 

 several rows of bristles ; these are frequently invisible to the 

 naked eye, but may be distinguished by the resistance they make 

 when the finger is passed along the body from behind forwards, 

 their points being directed backwards, in order to give the 

 animal a firm hold of the earth through which it is boring. The 

 head of these animals is not distinct from the body ; and they 

 have neither eyes, antennae, mandibles, cirrhi, nor external gills. 

 Their bodies, however, are distinctly divided into segments ; and 

 these are marked by minute spots on each side (Fig. 531, ), 

 which are apertures leading to small respiratory sacs, on the 

 walls of which the blood is submitted for aeration to the influ- 

 ence of the air or water received into them. This Order includes 

 only two principal groups ; the Earth- Worms and the Naids, 

 the former being inhabitants of the land, and the latter of the 

 water. 



843. The Earth- Worms, which nearly all belong to the genus 

 Lumbricus, generally live beneath the surface of the ground, either 

 perforating the dry soil, or burying themselves in mud, where 

 many of them lead a semi-aquatic life. When the Worm is 

 boring, it insinuates its pointed head between the particles of the 

 earth amongst which it penetrates like a wedge ; and in this 

 position, the anterior part of the body is fixed by the spines or 



