280 
The great difference in size between the sexual Ch. Limncei 
and the fissiparous larvae which abound through the year 
must not be overlooked. I believe that the larvae are smaller 
in the earlier months of the year, not when measured by the 
length of the chains of zooids, which are very long when 
found within the Limnaeus in February, but estimated by the 
breadth and the size of their setae. In September they appear 
to be getting bigger ; and at last, in October, the sexual 
forms are developed, measuring a maximum of one third of 
an inch for a head and sixteen fasciculi, as against one tenth 
of an inch in the larvae for two attached zooids, consisting 
each of a head and seven fasciculi, the posterior of which 
belong to new growing zooids. I may here also observe that 
the rate of production of new zooids probably varies with the 
abundance of nutriment, and that in some cases you may see 
as many as four or five segments (indicated by fasciculi) 
remaining attached behind a head without sign of separation ; 
but, as a rule, you will observe separation taking place after 
the third abdominal pair of fasciculi. 
Generative Organs. — Having above described the changes 
occurring in Ch. Limncei on assuming the sexual state, other 
than those relating to the genitalia themselves, I shall now 
describe these. In a specimen which consisted of two indi- 
viduals developing sexual organs and intermediate and ter- 
minal zooids in a state of growth, I endeavoured to fix the 
position of the generative organs in relation to the muscular 
septa which pass from the body-wall to the alimentary canal, 
and which in most Annelids agree with somites or segments 
in number, though I doubt if they do in the anterior region 
of Chaetogaster. I noted — 1, septum below the pharynx, 
followed by the commissural vessels (see former paper) ; then, 
2, a thinner septum, between which and 3, a similar septum, 
was a doubly pyriform mass of considerable size, cellular 
structure, and transparent. This double mass, or these two 
masses, were the testes, as was most clearly proved afterwards. 
They appear to have been mistaken by d’Udekem for “seminal 
receptacles” (PI. XIV, fig. 1), since he figures no testes, 
but organs to which he gives the above name in the position 
of what, there can be no doubt, are testes in Ch. Limncei. 
After septum 3 came two round masses of ova, one on either 
side the nervous cord ; below these the first pair of abdominal 
fasciculi ; the modified setae (hard pieces) had not yet appeared 
in this specimen. Then followed septum 4, between which 
and septum 5, which was a large, well-marked septum, was 
a pair of segment organs opening in front of a pair of fasci- 
culi, — the second abdominal fasciculi of this sexual form, the 
