J4 o ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



hairlik. microgamete can readily pass down the chinks and so 



rior. 



uiin to be mentioned as accessory reproductive organs the 



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



.til in the grooves between somites IX and X, and X and XI, 



which I'.Tin spin-nail pockets projecting forwards into the cavity of 



IX and X respectively, and which functionally serve as 



in which microgametes received from another worm are 



mtil needed lor the process of syngamy. When rilled with 



thev arc conspicuous by the brilliant white appearance 



reflection of light from the surfaces of the dense nuclear portions 



innumerable microgametes. 



The complicated arrangement of organs which has just been described 



r its object the production of zygotes new individuals in the 



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



the clitellum become active and produce a liquid secretion which 



. -; over the surface of the clitellum and there hardens to form an 



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



: indiarubbcr tubing. The worm next proceeds by writhing 



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



es 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 cocoon. 



K or macrogamete within the cocoon is fertilized by a micro- 



.nul one or more of the zygotes so produced proceeds to develop 



individual, the remainder degenerating. 



arthworm possesses a well-developed blood-system. Of the 



vessels the two most conspicuous are longitudinal the dorsal vessel 



'. more or less hidden amongst the yellow cells on the 



U e of the alimentary canal, and the ventral vessel (v.v) sus- 



i by a thin meml.rane underneath the alimentary canal. These 



!> lar-e hoop-like vessels in about five somites 



1 ^ ' ' ' " ; <' inuu end of the worm, which from their function 



the hearts. Connected with these main blood-vessels 



-els \\hirh divide into smaller and smaller 



inr1 ' "1 eventually into a network of extremely fine, thin- 



capillary blood-vessels in which the blood is brought into' 





