72 



Arenicolidae 



REPRODUCTIVE ORGANS. 



The reproductive organs are closely associated with the nephridta. 

 In the caudate species and in A. branchial the gonad is present 

 immediately behind the funnel of each 

 nephridium, except the first, and is a small, 

 ovoid or cylindrical mass of cells (PL XIV, 

 Figs. 48-50; PI. XV, Fig. 51) around the 

 anterior portion or the whole length of the 

 gonadial vessel, 1 which is a continuation of 

 the blood-vessel traversing the dorsal lip of 

 the funnel. The genital products are shed at 

 an early stage from the gonads into the 

 coelomic fluid, and by an examination of this 

 the sex of a given specimen is determinable. 

 There the small groups of spermatogonia 

 undergo repeated division and become succes- 

 sively spermatocytes, sperinatids and sperma- 

 tozoa, large discoidal masses of which are 

 present in the coeloin of mature worms. The 

 oocytes are shed from the ovary when they 

 are about 12 to 20 /z in diameter ; while in 

 the coelomic fluid their yolk is elaborated 

 and they grow to the definitive size. The 

 ripe ova of A. marina, pusilla, assimilis 

 and cristata 2 are not spherical but biconvex 

 (Fig. 42 B) ; the face of the ovum is either 

 circular or oval and its two diameters are 

 from 14 to 20 mm., while the third axis 

 of the egg is about *08 mm. The vitelline 

 membrane is comparatively thin, being 1 to 

 2/>tin thickness. The ova of A. branchialis 

 and ecaudata, which have a very stout 

 vitelline membrane, 5 to 6 /j, in thickness, 

 oval in shape and are each about 

 17 mrn. long and * 15 mm. broad (Fig. 42 c). 

 The reproductive organs of A. ecaudata are much larger and more 

 complex than those of the other species, and form the most striking 



1 There is a gonadial vessel on the first nephridium, but a gonad appears 

 never to be associated with it. 



2 Probably also those of A. glacialis and loveni, but eggs of these species 

 have not been available in a sufficiently good state of preservation to enable 

 the writer to determine their true shape. 



Fig. ^.-K. A. marina. Ripe sper- 



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