STRUCTURE OF ARENICOLA. 179 



cells which have kept to some extent apart from the forma- 

 tion of the embryo, certainly appear in association with the 

 mesoderm. 



The Lob- Worm — Arenicola piscatorum : — a type of the 

 marine Chsetopods or Polychseta. 



Habits. — On the flat sandy beach uncovered at low-tide, 

 the " castings " of the lob-worm are very numerous. There 

 the fishermen seek the worms for bait, and have to dig deep, 

 for the burrowers rapidly retreat far into the sand. The 

 burrows of the lob-worm are cylindrical tubes, lined by a 

 yellowish-green secretion, and the surrounding sand is often 

 discoloured by some change in which the organic juices 

 convert the mineral particles of iron into oxides. .The 

 tubes are at first vertical, and afterwards oblique or hori- 

 zontal. 



The lob-worm burrows like the earthworm — eating the 

 sand for the sake of the organic particles or sinall organisms 

 which it contains. The sandy castings, which pass from the 

 end of the food-canal, and are got rid of at the mouth of the 

 tube, fall into spiral coils. When getting rid of the castings, 

 the worm lies with its tail upwards and its head downwards, 

 or with its body bent like a bow ; when the tide comes in, 

 the mouth may protrude. The animal is able to turn in its 

 burrow. 



External Appearance. — The lob-worm varies in length 

 from eight inches to a foot, and at its thickest part is about 

 half an inch in diameter. There are three regions in the 

 body : — (a) the anterior seven segments, of which all but the 

 first have bristles j (b) the gill-bearing region of thirteen 

 segments ; (c) the thinner posterior part of variable length, 

 without either bristles or gills. The head-lobe is very small ; 

 there are no tentacles or eyes. Anteriorly a soft proboscis 

 is protruded. The anterior region is greenish-brown, the 

 middle region yellowish-red, the posterior region yellowish ; 

 but there is some variability. 



Skin and Muscles. — Each segment is marked by several 

 rings ; there are numerous warts on the posterior region. 

 Most externally lies the cuticle, then the pigmented epi- 



