1006 ELLIOT ROWLAND DOWNING 
The gonads are more restricted than the nephridia. As noted 
above, the first gonadial vessel in A. cristata never bears gonads, 
the sixth rarely and then degenerating cells only. In the other 
species the first nephridium never has a gonad. Here then is a 
case in which the genital cells show an evolutionary character, the 
tendency to restriction, more emphatically than the somatic cells. 
GENERAL STATEMENT 
A section through the testis of any of the Arenicolidae shows, 
ordinarily, a mass of cells of two or three sizes (fig. 7-9): These 
are the spermatogonia of successive generations. The larger ones 
lie adjacent to the blood vessel. At the periphery of the gonad 
spherical bunches of spermatogonia or occasionally single ones 
are seen to be loosening from the general mass preparatory to dis- 
charge into the body fluid (except in A. claparedii). In this 
fluid the further divisions of the cells result in the formation of 
hollow spheres of spermatogonia (fig. 10), the last generation of 
which grow to spermatocytes. These, by the customary two 
divisions, become the spermatids (fig. 11) the cells still adherent 
in the spherical masses, which are meanwhile however altering 
their shape and becoming saucer-shaped, in A. cristata (fig. 12) 
slightly biconvex in the other species, except in A. claparedii, 
in which the successive divisions occur in the testis and the sperm 
are discharged into the body fluid. These sperm masses are the 
spermatophores, (fig. 13). The body fluid of A. cristata is loaded 
with these spermatophores (or with the eggs) except for a short 
time just after the discharge of the sex products, and in the other 
species, except claparedii, for weeks before the eggs are deposited. 
There are present, of course, other elements, body cells, chlora- 
gogue cells, ete., but the dominant objects are the eggs and the 
spermatophores. Finally the tails of the sperm are stiff and are 
aggregated into conical masses (fig. 12). 
Toward the close of the breeding season, I have found, both in 
A. cristata and A. claparedii, exceptionally large spermatogonia 
discharged singly from the gonad, in addition to the customary 
masses described above. These develop in a somewhat different 
and highly instructive manner, as will be described later. 
