6 BRITISH WELL-WORMS.
graph, p. 19). Along with this we and also a remarkable
specialization of the nerve-cord in some of the segments—a
peculiarity which it shares with some of the Enchytraeids.
Leydig and others have further pointed out the existence of
valves in the dorsal blood vessel. While these appliances are
general in the earthworms they are rarely found in the lower
annelids, so that Phreoryctes in this respect is to be regarded as a
highly important type. Mr. Beddard in 1895 accorded to the
genus a position of great significance ; and though Michaelsen
has since reviewed the position, and my own discovery has yet
to be considered, I cannot refrain from summarizing the findings
of our greatest authority on this important subject.
The question which Mr. Beddard asks is :—"How far are
we justified, with our present knowledge, in separating the
aquatic from the terrestrial Oligochaeta ?" His answer shows
that in his judgment the genus under review forms in many
respects a very decided connecting link between the two.
"There are, undoubtedly, a certain number of points in which
all these (aquatic) forms agree to differ from the terrestrial
Oligochaeta. . . And there are, furthermore, a few points
which at present are peculiar to the aquatic Oligochaeta. We
will commence with the latter. Among all the Oligochaeta which
belong to Claparède's 'Limicolae,' the ova are of large size and
full of yolk ; this holds good, without a single exception, from the
smallest Enchytraeid up to so large a form as Phreoryctes.
The remaining point of difference concerns the structure of the
body wall. The longitudinal fibres consist of a single row of
deep fibres only (in the Limicolae or aquatic forms); this, how-
ever, does not characterize Phreoryctes, a genus which in other
characters occupies an intermediate position." Mr. Beddard
proceeds to examine the large and instructive group of worms
known as Enchytraeids, and adds—"The Enchytraeidae perhaps
resemble Phreoryctes more than any other group of the higher
Oligochaeta ; these resemblances, however, are not numerous,
and are confined to a few species. The most striking is the
existence in various species of Pachydrilus of the segmentally
arranged lateral outgrowths of the nerve-cord ; structures similar
to these appear to occur in Phreoryctes. Besides Phreoryctes, the
only Oligochaeta in which there are so few as four setae per seg-
ment, implanted singly, is Enchytraeus Monochetus." It was this
fact that led me to name the Essex worm Dichaeta, or the worm