THE ANATOMY. ALIMENTARY CANAL 55 



Which habitually live in or at the bottom of streams, lakes, etc. is the gizzard present ; 

 this fact might, and indeed has, led to the inference that its absence is to be accounted 

 for by the soft nature of the food. Probable though this hypothesis is, it seems to be 

 contradicted by the fact that, the gizzard is also absent in a number of terrestrial 

 Oligochaeta whose food is presumably quite the same as that of other terrestrial species 

 which possess a gizzard. Moreover a gizzard is wanting or rather represented only 

 by a rudiment in a species whose habitual food is harder than that of any other 

 Oligochaeta ; in Pontodrilus bermiidensis, which lives on the sea shore in coral debris 

 (with which its alimentary canal is always full) there is only the trace of a gizzard. 

 Microscolex too is a purely terrestrial form but it either has no gizzard or a degenerate 

 one. It is evidently therefore not safe to lay down any such general statement about 

 the cause of the presence or absence of the gizzard. In the greater number of earth- 

 worms the gizzard only occupies a single segment ; but the segment in which it is 

 found is not always the same ; in Luvibricus, for example, the gizzard is usually 

 in the xviiith segment; in Megascolex, on the other hand, the vith segment is 

 occupied by this organ. Very often there is more than a single gizzard ; when this 

 is the case the gizzards are in consecutive segments ; the genera Bigaster and 

 Dichoffa&ter have been so named on account of the presence of two gizzards ; there 

 are two gizzards also in Benhamia and three in the genus called by Fletcher 

 Perissogaster — a genus which is in the present work included in Bigaster. In the 

 genera Moniliga&ter, Pleionogaster, Hyperiodrilus, Heliodrilus, and one or two others, 

 there are a considerable number of gizzards— three to ten in number. 



The genus Perichaeta is remarkable for the fact that it is provided with only 

 a single gizzard, which nevertheless occupies two segments. It seems to be quite 

 possible that in this case there are really a pair of gizzards which have beconxe 

 intimately fused so as to form a single one. 



An important point to be noticed about the gizzard is that it may occur in any 

 segment or segments of the oesophagus ; it has no fixed position except of course 

 for the species or genus as the case may be. 



As to the histology of the organ, comparative researches are as yet wanting. It 

 is perhaps remarkable that the muscular tissue which enters into its formation is 

 precisely similar to that of other parts of the body and not striated; it so often 

 happens that the muscular tissue of organs of great muscular power is made up of 

 striated fibres that the negative fact — that this is not the case with the Oligochaeta — 

 is worth calling attention to. 



Calciferous glands. — Appended to the oesophageal region of the digestive tract 

 of many Oligochaeta are a series of glands which have been variously termed 



