ALIMENTARY CANAL 359 



it is modified into a gizzard. The gizzard is merely a portion of 

 the oesophagus with very much thickened muscular walls and with 

 a stout lining of chitin. It is not universally present among earth- 

 worms, and when present varies much in position. The rule is that 

 one gizzard only is present. In Digaster, as is implied by the name, 

 and in some other forms there are two in successive segments ; in 

 Trigaster, as the name also indicates, there are three gizzards ; in 

 Moniligaster and the Eudrilids Hyperiodrilus and Heliodrilus there 

 are four to six ; and a few other forms also have a considerable 

 number of gizzards. The calciferous glands are diverticula of 

 the oesophagus with folded and sometimes ciliated walls ; their 

 epithelium secretes calcareous particles, which are frequently of 

 crystalline form. Darwin supposed that this secretion was provided 

 in order to negative the humus-acids of the soil which is the food 

 of earthworma These organs are usually paired, but in the 

 Eudrilidae there are unpaired as well as paired glands ; the 

 unpaired calciferous glands lie ventrally. These glands are totally 

 wanting among the aquatic families, with the sole exception of 

 the Enchytraeidae. In a few of these there are either paired or 

 single glands of a very similar nature ; Dr. Michaelsen has 

 suggested that the function of these is rather absorptive than 

 secretory. From the median unpaired gland of Buchholzia arises 

 the dorsal vessel, which at first forms a sinus round the glandular 

 epithelium ; the epithelium, like that of the nephridia, is perfor- 

 ated by the ducts. In certain Oligochaeta there are some curious 

 modifications of the calciferous glands. In Stuhlmannia and a 

 few other Eudrilidae the oesophagus is beset with a larger number 

 of paired structures than in any other genera of the family, where 

 the calciferous glands are more limited in number. These glands 

 consist of a short tube lined with epithelium opening into the 

 oesophagus. Eound this is a mass of cellular tissue, but the out- 

 lines of the constituent cells are lost ; the whole is permeated 

 with abundant blood-vessels. This layer seems to be peritoneal, 

 and the entire gland seems to have lost its function as a secretory 

 organ, and to have taken on some function in connexion with the 

 vascular system. An analogous modification is to be found 

 among the Enchytraeidae. In certain forms there is a structure 

 known as the cardiac body ; this is a chord of cells lying in the 

 dorsal blood-vessel at the point where it springs from the 

 intestine. It is tempting to regard this cellular rod as being 



