30 OLIGOCHAETA 



to the nephi'idia and are not solid but hollow outgrowths of the septa ; they are often 

 rather racemose in form and are chiefly muscular with a lining and covering of cells. 

 The opening of their lumen into the cavity of the segment in front of that which contain^ 

 them is visible. Clapaeede has described and figured in the common earthworm solid 

 masses of cells enclosing a few muscular fibres and depending from the septa. Vejdovsky 

 states that similar bodies occur in RhyTichelmis and Tubifex. He suggests that they are 

 concerned with the growth of the septa. I found that the septal sacs of Acanthodrilus 

 were rich in Glycogen. The structure of the septal sacs in this worm and in Sutroa is so 

 like that of the sperm-sacs and egg-sacs that it is possible to see in them the remains of 

 a segmentally arranged series of sacs out of which the sperm-sacs and egg-sacs were 

 originally evolved. 



§ 4. Dorsal Pores. The coelom is placed in communication with the external medium 

 in a large number of Oligochaeta by a series of pores, one to each segment ; in addition 

 to these structures which are called the dorsal pores there is in a certain number, most 

 of the aquatic Oligochaeta, a single pore on the prostomium which is generally spoken 

 of as the head pore. The two coincide in the same species in the genus Fridericia 

 alone. The dorsal pores are never developed upon the first one or two segments of 

 the body, and the point where they commence is characteristic for the species ; in some 

 forms for example the first one will lie between segments iv. and v., while in others 

 the first pore lies altogether behind the cliteUum. The dorsal pores were considered 

 at one time to lead into sacs, the function of which was believed to be respiratory ; 

 it is now known that the pores are simply perforations of the integumental layers 

 just at the anterior boundary of the segment to which they belong; there is no 

 lining of epithelium as has been erroneously stated to be the case ; there is simply 

 a discontinuity of the muscular and epidermic layers where the pores exist. The 

 structure of these pores has been more particularly studied by Ude (3). In the 

 figure which this author gives of a section through a dorsal pore there are represented 

 a heaped up mass of peritoneal cells in the immediate neighbourhood of the pore; 

 the function of this is very doubtful. Dorsal pores are not present in by any means 

 all earthworms; they are absent for example in most of the Geoscolicidae, in many 

 if not in all Eudrilidae, and in a few species of Acanthodrilidae and of other families ; 

 among the lower Oligochaeta they are only found in a few species of Fridericia 

 (Enchytraeidae). Their structure in the latter has been studied by Vejdovsky and 

 MiCHAELSEN ; in these worms the pore is bordered by large round glandular cells on 

 each side; no such cells are visible in the case of the dorsal pores of earthworms. 

 We are at present completely in the dark as to the morphological meaning of these 

 pores. There seems to be no relation between them and any other organs; pores 



