THE ANATOMY OF PHORONIS AUSTRALIS. 151 
Each nephridium arises from some of the somatic meso- 
blast-cells. In Phoronis it is found essentially in the same 
position. It is true that the nephridia of the Sipunculus larva 
come to lie more ventrally than in Phoronis, where, indeed, 
they take up a dorsal position, though here they are in 
reality lateral. However, in both cases the point of origin 
is the same; but the secondary elongation of the body in 
Sipunculus is relatively not so great as it is in Phoronis; 
and if this elongation were to occur rather nearer the mouth 
in Sipunculus, or before the slight shifting of the nephridia 
takes place, they would lie much nearer the anus than they 
at present do. 
The young larva of Sipunculus is not a typical trocho- 
sphere: it is ciliated all over, and has one band of long cilia. 
This band is post-oral as in Actinotrocha; a pre-oral band 
does not appear to be present, although in a later stage there 
are a few long cilia on the pre-oral lobe which may represent 
it. The anus of the larva is nearly terminal, but is rather 
dorsally placed. 
The long axis of the body nearly coincides with the oro-anal 
axis, but during growth a secondary long axis becomes deve- 
loped, as it does in Actinotrocha; but whereas in the latter it 
comes about by the well-known invagination on the ventral 
surface, followed by an evagination, in Sipunculus it is 
merely due to the growth of the ventral surface taking place 
more rapidly than that of the dorsal surface. This elongation 
carries with it the intestine, just as is the case with Actino- 
trocha. Nosuchchange occurs in Polyzoa; the mouth and 
anus being close to one another from the first. 
In the full-grown larva there is a small pre-oral lobe, with a 
thickening which will form the supra-cesophageal ganglion 
asin Actinotrocha. There is a sort of collar behind the 
ciliated band which appears to be unrepresented in Actino- 
trocha. 
The arrangement of the alimentary canal is the same in 
both ; the csophagus opens into a stomach, whence the in- 
testine, more or less coiled in Sipunculus, passes forwards again 
