194 SEGMENTED WORMS OR ANNELIDA. 



proctodeum or "hind-gut" — which meets and fuses with the 

 archenteron, and forms the anus and a small portion of the posterior 

 gut. 



The mesoderm begins with the two primary mesoblasts already 

 described. These multiply and form mesoderm bands, which, insinuat- 

 ing themselves between ectoderm and endoderm, proceed to surround 

 the gut. At the same time, some of the mesoderm cells become 

 migratory, wander on to the head, and also surround the gut, before the 

 final trunk musculature is completed. The migratory mesoblasts of 

 the trunk appear to form a special larval musculature precociously 

 developed, in order to enable the embryo to manage the enormous mass 

 of albumen (absorbed from the capsule) with which its body is dis- 

 tended. The mesoderm bands grow in strength, and form a complete 

 ring encircling the archenteron. They then become two-layered, and 

 the two layers separate, the inner (splanchnic) cleaving to the gut, the 

 outer (somatic) clinging to the body-wall. The space between them is 

 the body cavity or ccelom. But as the separation of somatic and 

 splanchnic layers takes place, partitions are also formed transversely, 

 to become the septa which partition off the body cavity into a series of 

 segments. The cavity of the pre-oral region or prostomium differs some- 

 what from that of the others, being from the first unpaired, instead of 

 including two lateral cavities, one on each side of the gut. 



Type of Polych^eta. The Lob-Worm {Arenicola 

 marina). 



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

 the " castings " of the lob-worm or lug-worm are very 

 numerous. There the fishermen seek the worms for bait, 

 and have to dig quickly, for the burrowers retreat one to two 

 feet into the sand. The burrows are U-shaped tubes, lined 

 by a yellowish green secretion from the animal's epidermis, 

 and the surrounding sand is often discoloured by some change 

 which the secretion effects on the iron oxides and other 

 constituents. The tubes are at first vertical, afterwards 

 oblique or horizontal, and then turn vertically upwards again. 



The lob-worm burrows like the earthworm, not only 

 forcing the anterior part of its body onwards, but eating the 

 sand for the sake of the organic particles and small organ- 

 isms 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. It has been 

 calculated that in a year the average volume of sand per 

 acre thus brought up in castings is about 1900 tons, 

 representing a layer of 13 in. spread out over the 



