EARTHWORM. 315 



PLATE XII. 



EARTHWORM (Lumbricus terrestris). 



The fifteen anterior somites, numbered from before backwards, the ' prostomial segment ' 

 counting as the first.- 



THE integument has been divided, except in the prostomium, down 

 the middle dorsal line, and the greater part of the digestive tract has been 

 removed, together with the pseud-haemal vessels, so as to show the nervous, 

 nephridial, and reproductive organs. 



a i. Bilobed supra-oesophageal ganglionic mass ; giving off from either 

 outer angle a nerve which bifurcates very soon after its origin, and 

 distributes itself in the prostomium. 



a 2. Visceral or stomatogastric plexus of the right side. See p. 211, ante. 

 Described by Quatrefages, A. Sc. N. (3) viii. 1847, p. 36. 



a 3. Commencement of ventral nerve-cord. 



b. Pharynx, turned aside to the left, the right half of the organ, except 

 the small portion upon which the right stomatogastric plexus, a 2, 

 is seen, having been removed. 



c i. First nephridium, or ' segmental organ,' opening externally in so- 

 mite jv. Ordinarily the thickened muscular portion of the tube 

 opens externally in the somite immediately posterior to that in 

 which its internal funnel-shaped opening is situated. But as so- 

 mite iv is not cut off from somite iii by a perfect dissepiment 

 such as limits the somites after somite v inter se, the anterior 

 funnel-shaped termination is not in a different somite from the 

 one in which the coils of the gland are lodged. 



c 2. Nephridium similarly modified to c i. 



c 3, c 4, *: 5. Normal nephridia. The opening on to the exterior is 

 usually close to the inner row of setae, though it may vary con- 

 siderably, and even come to lie exteriorly and superiorly to the 

 outer row of locomotor spines. The funnel-shaped internal opening 

 is seen a short way from the outer edge of the nerve-cord, and 

 near the ventral surface in the somite anterior to that in which 

 the gland communicates with the exterior. In these organs the coils 

 of the posterior part, which is much the larger, are connected by a 

 mesentery-like lamina to each other and to the dissepiments of the 

 somites. 



d. Muscle passing up from one of the ventral muscles to attach itself to 

 the capsule of the supra-oesophageal ganglion to which it stands in 

 the relation of a powerful retractor. 



