152 ANNELIDA [CH. 



secreted, ' which is poured forth through a hole in the cuticle 

 opposite the end of the cell-neck and helps to keep the surface 

 of the worm moist. 



Beneath the ectoderm is a thin and hardly perceptible layer of 

 connective tissue forming a bed on which the ectoderm cells rest. 

 This foundation is called the derm is, and is included with the 

 ectoderm in the ordinary conception of the "skin." In contra- 

 distinction to the dermis the ectoderm is often spoken of as the 

 epidermis (Gr. ri, upon). 



Beneath the dermis comes a layer of circular muscles (3, Fig. 

 63), and beneath these again a much thicker layer of longitudinal 

 muscles. The circular muscles consist of a few layers arranged 

 to form rings round the section. The longitudinal muscles are 

 arranged very regularly, and in the section they have the form of 

 a series of feathers (5, Fig. 63), since the individual fibres appear 

 arranged in oblique rows between which tongues of connective 

 tissue extend, giving off lateral branches on which the fibres rest. 



Both sets of muscles are composed of muscle-cells. These are 

 long fibre-like structures pointed at both ends. Most of the proto- 

 plasm is differentiated into fine fibrillae, which indicate (see 

 p. 33) contractile power. In the centre of the cell is a patch of 

 unmodified protoplasm with a nucleus. The whole cell may be 

 compared to a myo-epithelial cell of Hydra in which the epithelial 

 part has diminished in size and the tail increased. Nor is this a 

 fanciful comparison, for the study of development teaches us that 

 the cell is actually derived in this way from the originall} 7 ' simple 

 cells of the wall of the coelomic sac or in the case of the circular 

 muscles from an ectoderm cell. 



The movements of the earthworm can be more easily under- 

 stood when the arrangement of the muscles is known. The 

 longitudinal muscles serve to shorten the body, and as the coelomic 

 fluid, like water, is practically incompressible, the diameter of the 

 animal must be increased, and thus the chaetae can be driven into 

 the sides of the burrow. On the other hand, the circular muscles 

 diminish the diameter of the coelom, and the contained fluid being- 

 forced to move in a longitudinal direction stretches the body out. 

 The holes in the septa equalise the pressure in the various segments 

 by permitting the fluid to escape from one into the next. 



Within the longitudinal muscles there is a layer of cells called 

 the parietal peritoneum (6, Fig. 63) which forms the immediate 

 wall of the coelom. The parietal peritoneum is composed of 

 flattened cells; the visceral peritoneum which forms the inner 



