140 ZOOLOGY FOR MEDICAL STUDENTS chap. 



hairlike mature microgamete can readily pass down the chinks and so 

 to the exterior. 



There remain to be mentioned as accessory reproductive organs the 

 two pairs of spermathecae (Fig. 67, sp). These are inpushings of the 

 body-wall in the grooves between somites IX and X, and X and XI, 

 which form spherical pockets projecting forwards into the cavity of 

 somites IX and X respectively, and which functionally serve as 

 receptacles in which microgametes received from another worm are 

 stored until needed for the process of syngamy. When filled with 

 spermatozoa they are conspicuous by the brilliant white appearance 

 due to reflection of light from the surfaces of the dense nuclear portions 

 of the innumerable microgametes. 



The complicated arrangement of organs which has just been described 

 has for its object the production of zygotes — new individuals in the 

 unicellular stage. When the worm is about to lay its eggs the gland- 

 cells of the clitellum become active and produce a. liquid secretion which 

 spreads over the surface of the clitellum and there hardens to form an 

 elastic membrane encircling the clitellar region of the body like a piece 

 of stretched indiarubber tubing. The worm next proceeds by writhing 

 movements to work itself backwards out of this elastic sheath. As the 

 sheath passes over somite XIV the macrogametes (commonly about four 

 in number) from the two receptacula are passed outwards so that they 

 lie between the sheath and the body. At the level of somites XI to IX 

 mature microgametes from the spermathecae — received it will be 

 remembered from another worm — are squeezed out into the same space. 

 Finally as the worm withdraws its head end from the elastic sheath the 

 ends of the latter close up and it forms a small lemon-shaped coeoon. 

 Each egg or macrogamete within the cocoon is fertilized by a micro- 

 gamete and one or more of the zygotes so produced proceeds to develop 

 into a new individual, the remainder degenerating. 



The earthworm possesses a well-developed blood-system. Of the 

 vessels the two most conspicuous are longitudinal — the dorsal vessel 

 (Fig. 63, d.v), more or less hidden amongst the yellow cells on the 

 dorsal surface of the alimentary canal, and the ventral vessel (p.v) sus- 

 pended by a thin membrane underneath the alimentary canal. These 

 vessels are connected by large hoop-like vessels in about five somites 

 (VII to XI) towards the front end of the worm, which from their function 

 are known as the hearts. Connected with these main blood-vessels 

 are numerous smaller vessels which divide into smaller and smaller 

 branches and lead eventually into a network of extremely fine, thin- 

 walled, capillary blood-vessels in which the blood is brought into 



