POLYCH-ETA FROM THE N.E. PACIFIC. 983 
structures which are generally stated to be tentacles. They are 
very small flat organs, lying on each side of the prostomium, and 
generally covering the eyes. Claparéde showed, in the case of 
P. socialis, that they contained two or three thin capillary sete. 
This circumstance is easily verified in such species as I have 
examined, and it leads me to suggest that we are in error 1n 
regarding these structures as tentacles. They are much more 
probably the reduced and modified notopodia of the peristomial 
segment itself, 
It may be of some advantage to dwell on this point at greater 
length. In Annelids the peristomium 1s the segment behind the 
prostomium, and it can be generally recognised and homologised 
throughout the group. This fact is, however, due more to its 
position than to any morphological characters which distinguish 
it from those sueceeding, and it is generally recognised as the 
most anterior of the trunk-segments, which has been considerably 
modified in connection with the mouth. With regard to its 
identity with the other segments of the trunk, Goodrich says :-— 
“‘ Gareful modern researches (Vejdovsky, Wilson, etc.) have shown 
that in Oligochetes the peristomium exhibits the essential charac- 
ters of a true segment. It develops as a region surroun ding the 
mouth, in which are formed a pair of mesoblastic somites which 
become hollowed out to form the ccelom; a ganglionic thickening 
is produced ventrally, which soon fuses with that of the succeeding 
segment; a nephridium (head kidney) is developed. In the 
Polychetes—in some cases, at all events,—it has been shown that 
a pair of somites are formed in the peristomium, become hollowed 
ont, and even give rise to peritoneal funnels. Nephridia are 
almost invariably developed in this segment. In Polychetes, 
moreover, a pair of lateral appendages are often developed, though 
they generally become highly modified. In fact, it becomes 
evident, when we examine the development and the adult structure 
of the peristomium in the various groups of the Annelids, that it 
is really a metamere strictly comparable to the posterior segments, 
even when much modified owing to its position at the anterior 
end of the animal.” 
In the Syllids, the Polynoids, and other groups, the peristomium 
consists of a segment which bears on each side a dorsal and 
a ventral cirrus. That these are the remains of a typical para- 
podium, in which the notopodium and neuropodium with their 
setee have disappeared, is shown by a number of cases amongst 
the Polynoids, which could probably be duplicated in other 
families, where the first segment carries not only cirri but also 
sete. In Polynoé extenuata, described by Claparéde, the peri- 
stomium shows an aciculum and a couple of setae. In Pontogenia, 
Sthenelais, Sigalion, and in Palmyra amongst the Palmyride, the 
peristomium possesses a notopodium with several sete, and only 
differs from the succeeding segments in the absence of neuropodial 
sete. 
These cases show that the peristomium may be an almost 
