POST-LARVAL STAGE OF ARENICOLA MARINA. 51 
abnormal position of the anterior margin of the somite. But such is 
not in reality the case; dissections of the adult, as well as observa- 
tion of the nephridia of this present stage, indicate that the chete, 
and thus the ridge and groove, are in the middle of the somite, for 
in the anterior regions of the adult are certain complete septa, the 
last of which is placed midway between the third and fourth bundles 
of cheetee, the next anterior midway between the second and third 
bundles. The anterior end of the nephridium in the somites con- 
taining these organs lies about midway between consecutive bundles 
of chaetze—where, in fact, the septum would occur. I am unable to 
distinguish the septa in the present mounted specimen. 
Of internal structures I will draw attention more particularly to 
the vascular system. Wirén (loc. cit., p. 38, et seq.) has shown that 
in the adult Arenicola the dorsal and ventral blood-vessels are merely 
local enlargements of a continuous perienteric sinus, from which the 
axial portions are slightly nipped off. Now at this post-larval stage 
I find no sinus. Both the dorsal and the ventral vessels are quite 
distinct from the wall of the gut, as the camera drawing (fig. 4) shows. 
From these axial vessels branches pass right and left to the wall of 
the gut (diagrammatically shown in fig. 8), where they subdivide 
to a slight extent, and give rise to a but feebly developed plexus 
below the ccelomic epithelium. 
I have already, in discussing the perienteric blood-sinus of certain 
earthworms,* referred to the improbability of its beg a primitive 
feature, as is sometimes held. Here in Arenicola, at any rate, we 
have a network preceding ontogenetically the sinus of the adult. 
Other vessels pass to the gill as shown; the dorso-branchial vessel 
passes straight to the gill, curves round at the apex of the latter, 
and after two or three twists passes to the ventral vessel. Another 
vessel passes to the nephridium, but I have not traced its origin. 
The blood-vessels are covered by ccelomic epithelium, which, in the 
case of the ventral trunk and the ventro-branchial vessel, consists of 
rounded cells with brownish granular contents and a round nucleus ; 
they are, in fact, chloragogic cells: elsewhere the ccelomic epithelium 
is flat. 
In the body-wall the circular muscles are already present, and the 
epidermis rests directly upon them—the subepidermic tissue 
described by Wirén has not yet made its appearance. 
The longitudinal muscles are interrupted at three points, ventrally 
at the nerve-cord and dorso-laterally at the level of the dorsal 
chetz. The oblique muscles so characteristic of Polycheta are 
attached at these same points (see fig. 4). 
* Quart. Journ. Mic. Sci., xxxiv, 4 New English Genus of Aquatic Oligochzeta 
(Sparganophilus). 
