COMMON EARTHWORM. 



203 



of Calcium carbonate in the form of rhombohedra (especially in the first pair), or of 

 minute spherules. The Calcium carbonate has a chemical action, not a mechanical, 

 as supposed by Claparede, and partly neutralises the acids of the humus swallowed 

 by the worm, partly acts on the humus itself. 



The dorsal wall of the stomach-intestine is invaginated in nearly all terrestrial 

 Oligochaeta to form a ridge or typhlosole. This structure is absent in Megascolex, 

 and in the aquatic Criodrilus and semi-marine Pontodrilus, which in many respects 

 resemble the terrestrial forms. The ridge is complicated anteriorly, simple pos- 

 teriorly, and consists of all the layers entering into the wall of the intestine. In 

 some cases (Urochaeta^ Allolobophora cyanea, &c.) a longitudinal supra-intestinal 

 vessel runs along the cavity of the typhlosole, but in Lumbricus its walls are supplied 

 by vessels derived from the dorsal longitudinal vessel. It is kept invaginated, 

 according to Claparede, by muscles passing across from one to the other side in 

 each somite. The epithelium covering it internally is often found enlarged with 

 smaller cells between the bases of the large cells. The large cells are probably 

 thrown off into the cavity of the tract and there resolved. The same changes have 

 been noted in the intestinal epithelium of Rhynchelmis. 



The digestive tract ends with a short thin-walled rectum, which is lined by 

 cuticle and extends only through a single somite. Thie anus is dorsal in Criodrilus, 

 and there are in this worm at least seven somites with an embryonal nerve-cord 

 traceable behind it. 



A longitudinal dorsal and ventral mesentery are present in Criodrilus. The 

 dorsal is aborted in other instances, but the ventral usually persists. 



The contents of the oesophagus are acid ; of the stomach-intestine alkaline. 

 There is a diastatic and tryptic ferment, and a peptic as well, but the latter must 

 be inactive in the living animal owing to the alkalinity of the stomach (Krukenberg). 



The vascular system of the Earthworm consists of a median dorsal vessel, of a 

 supra-neural (= ventral) vessel, and three neural vessels, two lateral and one sub- 

 neural 1 . The dorsal, supra-neural, and sub-neural vessels branch anteriorly and 

 anastomose on the pharynx. In the region of the oesophagus six pairs of dilated 

 and pulsatile vessels or 'hearts' pass from the dorsal to the supra-neural 

 vessel. A small seventh pair gives origin to a lateral oesophageal vessel on each 

 side which runs forward to the pharynx. The supra-neural vessel is connected from 

 place to place with the capillary network in the walls of the intestine, and by this 

 means indirectly to the dorsal vessel. A vascular loop unites the dorsal to the 

 sub-neural vessel in each somite of at least the intestinal region. The integument 

 and nephridia are provided with a rich supply of capillaries. The blood supply of 

 the latter is said to be connected with the supra- and lateral, neural vessels ; of the 

 former, with the dorsal and sub-neural vessel. But there is some uncertainty about 



1 Howes figures in his Atlas of Practical Elementary Biology (PL xii. fig. 2) a sub- or infra- 

 intestinal vessel applied to the inferior aspect of the intestine. Such a vessel exists in some Poly- 

 chaeta and Oligochaeta, but not in the common Earthworm, so far as I can find from actual observation, 

 by dissection of fresh specimens and of mounted sections. Ben ham also appears to have failed to 

 find it (Q. J. M. xxvi. p 253). It seems to me that the dark streak on the inferior aspect of the 

 intestine which looks like a vessel is due to the attachment of the ventral mesentery in which the 

 supra-neural or ventral vessel is suspended, and the consequent absence of chloragogen cells. Dark 

 lines due to the same cause mark the attachments of the septa to the intestine. If the chloragogen 

 cells are removed by a scratch, the scratched spot has a similar dark appearance. 



