THE ANATOMY. ALIMENTARY CANAL 69 



means necessarily imply a morphological difference ; and Michaels en is far from 

 suggesting directly any such difference ; the fact however that he gives them a different 

 name would perhaps lead to the inference that there was some difference of structure ; 

 MiCHAELSEN does not in his descriptions of the minute structure of these glands (12) 

 show any differences of importance from the paired calciferous glands of the Eudrilidae 

 and of other Earthwoi-ms ; there is however a difference which I have noticed and 

 described in Heliodrilus and Hyperiodrilus ; in these two genera, and in all probability 

 in others, the periphery of the glands is occupied by a network of tubes whose lumina 

 are intra-cellular ; the rest of the gland shows only inter-cellular lumina. This is 

 however in my opinion not a matter of great importance ; the excessive folding of 

 the epithelium of the pouch becomes at length so complex that the lumen inevitably 

 becomes intra-cellular, the folds get to be smaller than the length of a single cell; 

 this at least is my explanation of the matter. 



As to the supposed difference of function — it does not exist in every case — I have 

 found that in Eudrilus the unpaired glands secrete calcareous particles entirely similar 

 to the particles secreted by the paired gland, and so has Udb (4). There are so many 

 instances among the Oligochaeta of organs being paired in one genus and unpaired in 

 another, that a difference of this kind cannot be looked upon as of much importance ; 

 the various parts of the generative apparatus are sometimes unpaired, though as a rule 

 paired; the contrary occurs with the dorsal vessel which is as a rule unpaired, but 

 occasionally paired ; even the brain is more or less completely divided into two halves. 

 The actual fact as to the pau-ed or unpaired character of the glands is not therefore in 

 my opinion a matter for serious consideration in deciding upon their homology. We 

 may fairly regard them as structures which are serially homologous. 



These organs, the paired and the unpaired, consist of a diverticulum of the 

 oesophagus, which is lined by epithelium continuous with that of the oesophagus ; 

 as a rule this epithelium is rather different in appeai'ance from that whence it has 

 been derived ; the minute structure of the glands has been studied in Lumbricus 

 (CLAPAEiDE, 1), Pontoscolex (Peeriee, 5), Ocnerodrilus (Beddaed, 20, and Eisen, 1), 

 Eudrilus (Beddaed, 62), Heliodrilus (Beddaed, 54), Alvania (Beddaed, 39), and in 

 a few other types; Ocnerodrilus is the least complicated of these. In that genus the 

 single pair of glands have a capacious lumen which is only moderately divided up by 

 internal folds ; the cells lining the diverticula are ciliated, as are those of that part of 

 the oesophagus at least which follows the apertures of the glands into it. Among the 

 Eudrilidae the subdivision of the lumen by numerous anastomosing folds has reached its 

 highest point ; and here as in many other species the epithelium is not ciliated ; in 

 Pontoscolex the epithelium is remarkable for being a low flattened epithelium ; it is 



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