NERILLA AN ARCH [ANNELID. 
417 
and Histriobdella. For in Archiannelids, as in Oligochsetes, 
specialisation seems to have led to a restriction of the gonads 
to fewer and fewer segments. In Polygordius and Proto- 
drilus a large number of segments are fertile, in Saccocirrus 
only the middle region produces gonads, in the male Nerilla 
three segments, and in the female one; finally, in Dinophilus 
and Histriobdella a single pair of gonads is present in both 
sexes. Accompanying this reduction is a corresponding dimi- 
nution of the number of ducts. 
The Body-Cavity in the Archiannelids. 
Nerilla throws a new light on the structure of the aberrant 
Dinophilids and Histriobdellids. It has often been held 
(Harmer [10], Schimkewitsch [20], Shearer [22]) that the 
general body-cavity of Dinophilus is not of coeloinic, but of 
blastocoelic or of haemocoelic origin ; that the coelom is repre- 
sented by the cavity of the ovarian and testicular sacs alone. 
Several years ago (6) I adopted this view myself, and Shearer 
has recently applied it to Histriobdella (23), but it now seems 
to me much more probable that these two genera are as 
specialised in the development of the coelom as they are in 
many other characters. Already Foettinger has described a 
coelomic cavity, and a ccelomic epithelium, extending through- 
out the anterior segments of Histriobdella (4), and Salensky 
(19) has drawn attention to the secondary invasion of the 
coelom by a network of “ coelenchym ” in Saccocirrus and 
Protodrilus. To put the matter shortly, the course of 
specialisation seems to have been as follows: In Dinophilus 
and Histriobdella the gonads have become restricted to one 
segment, the cavity of which is shut off by complete septa, and 
so forms a sac (“ovary,” or “testis”). The septa in other 
regions have broken down, and the coelom lias become in- 
vaded by a secondary ingrowth of mesoblastic tissue. Already 
in Nerilla, as shown above, the anterior septa have almost 
completely disappeared, and a delicate network of mesen- 
chymatous fibres extends in all directions through the coelom. 
