ANATOMY AND CLASSFFICATION OP THE ARENICOLID^. 453 



the brain and in the thickness of the body-wall. The appear- 

 ance of these brown granules is veiy similar to those of the 

 secretory portion of the nephridia^ but until the micro- 

 chemistry of each set of granules is known, it is impossible 

 to do more than suggest that the skin may in Arenicola act 

 as an excretory organ. In A. marina, A. Claparedii, and 

 A. cristata the furrows between the annular ridges are free 

 from pigment, and contain few or no gland cells. 



The remaining elements of the epidermis are sensory and 

 nervous, and are dealt with under their respective sections. 



Beneath the epidermis there is a slight connective tissue 

 which accompanies the nerves, the subdermal extensions of 

 the coelom, and the superficial capillaries, which (e. g. in 

 A. marina) may penetrate between the bases of the epidermal 

 cells. 



6. Musculature. 



In all species the muscles of the body-wall are similar to 

 those of A. marina. Immediately beneath the epidermis 

 there is a layer of circular muscles, and beneath this are the 

 bands of longitudinal muscle-fibres, which are well developed 

 in A. Grubii and A. ecaudata. 



Each bundle of notopodial set^e is enclosed in a sac, to 

 the inner end of which are attached (1) a single retractor 

 muscle strand, which is inserted into the body-wall at the 

 sides of the nerve-cord ; (2) six to ten protractor muscles, 

 which are inserted in the sides of the body at the level of 

 the setal sac. 



The musculature of the buccal mass is well developed in 

 A. Grubii and A. ecaudata, forming an almost complete 

 sheath round the phai'ynx. The muscles forming* this sheath 

 arise from the longitudinal layer in the region of the first 

 diaphragm, and are inserted into the anterior region of the 

 proboscis. They are not quite so well developed in A. 

 cristata and A. Claparedii. 



The muscular fibres are essentially composed of a central 



