PIXELL—POLYCHATA OF THE INDIAN OCEAN 73 
includes forms with opercular spines on which hooks are never present laterally and may 
be absent altogether (figs. 4 and 5). 
Except with regard to the shape of the operculum these genera resemble the genus 
Serpula very closely and are for this reason by some authors regarded as sub-genera only. 
It must be remembered however that the operculum, which is not generally a satisfactory 
organ on which alone to base distinctions, is here developed as a highly specialised organ 
of offence as well as of defence. Owing to the way in which the spines can be separated 
and then brought together by the action of muscles (figs. 2a and 6), the central cavity 
forms a highly efficient cage in which no doubt small animals are held prisoners and killed 
by coming in contact with the sharp central spines. Small Crustacea were frequently 
found imprisoned in this way, and one specimen of Hydroides heteroceros was found 
grasping with its spines a small Serpulid in its tube which had evidently just been torn 
from its substratum. 
The general characteristics of the genera Hydroides and Hupomatus then are : 
1. Collar setze of bayonet shape with two conical processes at base of blade. 
2. Uneini with a few coarse teeth—the most anterior one being larger than 
the rest. 
”) 
3. Abdominal setze trumpet-shaped (‘‘cornet comprimé” Gravier, |“ Spateln 
Marenzeller). 
The two genera can easily be distinguished by their opercula, and upon the 
apparently constant characteristics of these opercula the annexed scheme of classification 
Is suggested, 
[#. norwegica*, Gunn 
more than one pair of lateral processes H, multispinosa, Marenz. 
la . parvus, Treadwell 
[ H., homoceros, n. sp. 
not terminal lz bispinosa, Bush 
H 
Alspunes alike . erucigera, Morch 
(homoceros) | 
one pair of lateral 
oe H. lunulifera, Clpd. 
eae ae with jpuogesses Cully terminal Hl ee | 
lateral processes (Eucarphus Morch) ER MAE IO ear 
1. dirampha, Morch 
benzont, Morch 
. heteroceros, Grube 
bifurcatus, 2. sp. 
. monoceros, Gray. 
minax, Grube 
. albiceps, Ehrb. 
one spine only without lateral processes 

Spines of 2 kinds 
(heteroceros) | 
only one spine with lateral processes 
Sass SSSSISSUSSESSILSS 
NN 
. uncinatust, Phil. 
. elegantulus, Bush 
. protulicola, Bened. 
. spongicola, Bened. 
. dipoma, Schm. 
. exaltatus, Marenz. 
. Spinosus, N. sp. 
Hupomatus Spines all alike (homoceros) 
opercular spines with- 
out lateral processes 
SO 
SESS SSESSSS SSIS] 
Spines not all alike (heteroceros) 
X 
* According to Saint-Joseph H. pectinata and H. tripanon are only varieties of H. norwegica [46, p. 247], 
so also apparently are H. (Hupomatus) elegans, Haswell [26 and 27] and H. abbreviata, Kr [29]. 
+ The spines vary from 6—11. Bush [8, p. 498] separates Ehler’s specimens [15] with 11 spines as 
#. floridanus ; this seems to be unnecessary and the forms #. gracilis and H. humilis, described by this author, 
are possibly also #. wneinatus. EH. (Hydroides) dianthus, Verrill, seems to be no more than a variety. 
SECOND SERIES—ZOOLOGY, VOL. XVI. 10 
