84 MANUAL OF ANIMAL BIOLOGY 



project into the coelomic cavity. The basal end of each seta is 

 covered over by a number of muscle fibers which are attached to 

 the near-by body wall. The arrangement and attachment of the 

 muscle fibers to the setae is of such a nature that the ends of the 

 setae, which project externally through the body wall, can be 

 pointed either anteriorly or posteriorly by a contraction of the 

 proper fibers. This permits of their use in locomotion. (W. f. 61.) 



B. Organ Systems 



It has been noted that the body plan of the Earthworm may be 

 described as a tube within a tube ; the outer tube being the body 

 wall, the smaller enclosed tube the alimentary canal, and the space 

 between the body wall and the alimentary canal constituting the 

 body cavity, or coelom. A study of the embryonic development 

 of the Earthworm shows that the coelom is formed by a splitting 

 of the mesoderm into an outer layer, which is in close contact with 

 the outer ectoderm and forms the main portion of the body wall, 

 and an inner layer, which is in close contact with the functional 

 endoderm of the alimentary canal. Thus the coelom is com- 

 pletely lined with mesoderm. The coelom develops in essentially 

 the same way in all higher types and, although subject to consider- 

 able structural variation, it is an important feature of the triplo- 

 blastic animals, and particularly so in the Vertebrates. 



The coelom of the Earthworm is not a single open space running 

 from the anterior to the posterior end of the body, but is divided 

 into as many compartments as there are segments in the body. 

 These divisions of the coelom are due to membranous dividing 

 walls, or septa, 1 which are attached both to the body wall and to 

 the alimentary canal. Each of the segmental grooves, which can 

 be seen on the external surface of an Earthworm, marks the at- 

 tachment of a septum to the body wall. (W. f. 59, A, B, C.) 



Structure of the Body Wall. A microscopical examination 

 of a prepared transverse section through the body wall of the Earth- 

 worm shows the following arrangement of tissues. (W. f. 61.) 



The outer layer consists of a very thin, transparent membrane, 

 known as the cuticle. This is not a cellular layer, but is formed 

 as a secretion by the epidermal cells which lie directly under- 

 neath. Most of the epidermal cells contain pigment, especially 

 those on the dorsal side of the animal, with the result that this 



1 Singular, septum. 



