THE EARTHWORM 21 



by a relatively considerable space, the body-cavity or coelom, 

 in which other organs lie. The coelomic space, however, is 

 not continuous, but is broken up into a series of chambers by 

 muscular partitions or septa passing right across the body. 

 Each septum corresponds to an intersegmental groove, being 

 inserted on the body-wall all round the line marked out by 

 the groove, and is perforated by the gut. Thus the septa 

 divide the interior of the body into a number of compartments 

 which correspond exactly with the external rings, and since, 

 as we shall see, many of the organs are repeated in each 

 segment, and the segments lie one behind the other along 

 the longitudinal axis of the body, the earthworm is called 

 a metamerically segmented animal, and the individual seg- 

 ments or somites are sometimes called metameres. The septa, 

 however, do not exactly correspond with the external annula- 

 tion, for there are no septa between segments i — 2 and 

 2 — 3, and the septum between 3 — 4 is frequently absent, or, 

 if present, is represented by mere muscular slips. The septum 

 between 17 — 18 is also incomplete. 



The gut, as has been said, pursues a straight course from 

 the mouth to the anus, but anteriorly to segment 18 it is 

 variously modified. The mouth leads into a short but 

 rather wide buccal cavity with thin walls. Following upon 

 this is a stout pear-shaped pharynx, with thick muscular 

 walls extending as far back as the sixth or even the seventh 

 somite when the animal is moderately extended. In reality, 

 however, the pharynx lies entirely in front of the septum 

 separating the fifth from the sixth segment, both this and 

 the preceding septum being carried backwards by the posterior 

 projection of the pharynx. Anteriorly the pharynx begins in 

 the third segment, and it must therefore be spoken of as 

 extending from the third to the fifth segments. The walls 

 of the pharynx are attached to the body-wall by numerous 

 muscular strands, which, for the most part, run diagonally 

 backwards, the most posterior strands perforating the septa, 

 and extending as far back as the eighth segment (fig. 4, A). 

 The pharynx is succeeded by the oesophagus, a narrow straight 

 tube extending to the fourteenth segment. There are three 

 pairs of apparent pouches on the sides of the oesophagus, 

 one pair in the eleventh and two pairs in the twelfth somite. 

 These are usually known as the calciferous glands. In reality. 



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