xin EXTERNAL CHARACTERS BODY-WALL 349 



open directly into the body-cavity, and are mere perforations of the 

 body-wall, not tubes lined by a special layer of cells. Professor 

 Spencer of Melbourne 1 has observed a giant earthworm (Megasco- 

 lides australis) of Gippsland which, when held in the hand, spurts 

 out, to a height of several inches, the fluid of the body -cavity 

 through its dorsal pores. The burrows, he remarks, are coated 

 over with the same fluid, which is regarded by him as a lubricant. 

 This, however, considering that the glandular cells of the epidermis 

 can secrete a mucous fluid, seems to be an expensive use to which 

 to put the important fluids of the interior of the body. It is 

 more probable that the dorsal pores are a means of getting rid of 

 waste products. Lim Boon Keng 2 suggests that the coelomic 

 fluid possesses a bactericidal function. The dorsal pores are 

 missing in many earthworms, and without exception in those 

 Oligochaeta which live in water : but these latter worms have a 

 pore upon the head, which appears to be wanting in the earth- 

 worms. Dr. Michaelsen has thought that the head-pore serves 

 to relieve the brain from undue watery pressure to act, in fact, 

 as a kind of safety-valve for the liberation of superfluous fluid. 



In some foreign worms the pores of the reproductive ducts 

 are conspicuous external features (Fig. 197); even in our 

 British species the turgescent male apertures upon the fifteenth 

 segment are sometimes quite obvious. 



Structure of the Body-Wall. The body- wall consists in all 

 Oligochaeta of three recognisable sheets of tissue. Outside is 

 the epidermis, which always consists of a single layer of cells, 

 except in the clitellar region of earthworms. It is a point 

 of difference between the aquatic genera and the terrestrial 

 forms that in the former the clitellum is only one cell thick, 

 while in the higher Oligochaeta it is made up of more than one 

 layer of cells. The epidermis i ciliated only in the genus 

 Aeolosoma, and there only on the prostomium. It secretes a 

 thin layer of chitin, which is defective opposite to the glandular 

 cells, and becomes therefore perforated by numerous pores. 

 The structure of the epidermis of LumbriccH has been studied 

 by Cerfontaine, whose recent account 3 of the same is the fullest 

 and most accurate that exists. 



Underneath the epidermis comes a layer of circular muscle-fibres, 



1 Trans. Roy. Soc. Victoria, vol. i. 1888, p. 1. 

 2 Phil. Trans, clxxxvi. 1895, A, p. 383. 3 M4m. war. Ac. Behj. lii. 1890-93. 



