188 SEGMENTED WORMS OR ANNELIDA. 



in the region of the gullet by five or six pairs of contractile 

 vessels or "hearts." The precise path of the blood is not 

 known, but the distribution of vessels to skin, nephridia, 

 and alimentary canal is readily seen. 



Respiration is effected by the distribution of blood on 

 the general surface of the skin. 



Excretory system. — There is a pair of nephridia in each 

 segment except the first four. Each opens internally into 

 the segment in front of that on which its other end opens 

 to the exterior. They remove little particles from the body 

 cavity, and get finer waste products from the, associated 

 blood vessels. Nephridia occur in many animals, in most 

 young Vertebrates as well as among Invertebrates, but they 

 are never seen more clearly than in the earthworm. When 

 a nephridium is carefully removed, along with a part of 

 the septum through which it passes, and examined under 

 the microscope, the following three parts are seen : — 

 (a) An internal ciliated funnel ; (b) a trebly coiled ciliated 

 tube, at first transparent, then glandular and granular ; and 

 (c) a muscular duct opening to the exterior. Minute par- 

 ticles swept into the ciliated funnel pass down the ciliated 

 coils of the tube, and out by the muscular part which opens 

 just outside of the ventral bristles. The coiled tube con- 

 sists in part at least of a series of intracellular cavities, that 

 is to say, it runs through the middle of the cells which 

 compose it ; the external muscular portion arises from an 

 invagination of skin. 



Reproductive system. — Like all Oligochsetes, the earth- 

 worm is hermaphrodite and the organs complex. The 

 complexity is produced by the specialisation of certain of 

 the nephridia to form genital ducts and accessory organs, 

 and by the presence of chambers (seminal vesicles) con- 

 nected with the testes, formed by the shutting off of portions 

 of the body cavity. 



The organs in the earthworm are difficult to dissect, and 

 differ considerably in old and young specimens. 



(a) The Male Organs consist of two pairs of testes, three 

 pairs of seminal vesicles, and paired vasa deferentia. 



(i) The testes, flattened lobed bodies, about ^ in. in size, 

 arise from proliferations of the peritoneal lining of the body 

 cavity, and are invested by a delicate membrane derived 



