THE EARTHWORM 



27 



can best be seen in transverse sections (fig. 7), when it can be 

 seen that it is divided into right and left halves by a median 

 vertical partition of connective tissue. The lower and lateral 

 parts of each half are occupied in the ganglionic swellings by 

 nerve ganglion cells, and the middle and upper parts by fine 

 longitudinally disposed nerve-fibres. In the upper part of the 

 cord there are three so-called giant-fibres, looking like three 

 large tubes. Each consists of a central bundle of nerve-fibres 

 enclosed in a double sheath, the bundles being direct pro- 

 longations of certain large nerve ganglion cells. In the adult 

 worm the nerve cord lies wholly in the body-cavity, but it will 

 be shown that in development it arises from the ectoderm in 

 the form of two bands of cells which unite in the middle line 

 and sink below the epidermis. 



The blood- vascular system of Lumbricus herculeus com- 

 prises five main longitudinal trunks with their branches and 

 communicating vessels. (i) The dorsal or supra-intestinal 

 trunk lies just above the gut, extending from the pharynx to 

 the hinder end of the body ; anteriorly it breaks up to form 

 a plexus of branching vessels on the walls of the pharynx. 

 The course of the blood in the supra-intestinal vessel is from 

 behind forwards. (2) The 'sub-intestinal trunk lies between 

 the gut and the ventral nerve cord, being suspended from the 

 former by a vertical fold of connective tissue. In it the blood 

 runs from before backwards. (3) The sub-neural trunk lies 

 close under the ventral nerve cord ; in it the blood runs from 

 before backwards. (4 and 5) The lateral-neural trunks are 

 closely applied one to each side of the ventral nerve cord. 

 The principal connections between and branches from these 

 vessels are shown in the diagrams A, B, C, D, fig. 6. The 

 supra-intestinal trunk communicates with the sub-intestinal 

 trunk by five pairs of dilated contractile vessels, one pair in 

 each of the segments 7 n. These enlarged vessels, called 

 hearts, are conspicuous objects when the worm is opened from 

 above. The blood running forwards in the supra-intestinal 

 vessel is driven by the contraction of the hearts into the sub- 

 intestinal vessel, and there flows in the reverse direction. 

 Each heart gives off posteriorly a branch which passes to the 

 septum behind and supplies branches to it. 



The blood in the sub-intestinal vessel is carried outwards 

 by two vessels in each somite- Of these one goes to the 



