REPORT ON THE POLYZOA. 
xix 
wider peduncular process, which usually, but not always, corresponds with a sinus or 
notch in the lower border of the orifice. The former kind may be termed “ truncate ” 
and the latter “ pedunculate opercula.” 
2. Structure. — Some are composed of a continuous rigid, chitinous substance, but in 
the majority of cases the operculum is constituted of a thinner or stronger membrane, 
supported by a thickened chitinous border, to which are often added lateral rods or 
processes, or a more or less complicated framework ; and they may be divided into ; — 
a. Complete. — In which the chitinous rim is continuous all round including the 
lower border. 
b. Incomplete . — When the lower border remains membranous or is continuous, 
without distinct interruption, with the membranous endocyst of the 
zooecium. 
3. Articulation of the Opercula . — The mode in which the opercula are articulated 
varies very considerably. In those cases in which the zocecia are completely calcified, as 
in the Escharina generally, the operculum is directly articulated or attached to the sides 
of the orifice, usually at points near its inferior border, but sometimes higher up. The 
articulation is effected by an elastic fibrous ligament as it may be termed, which is generally 
inserted into a notch in the border of the operculum, but in some few cases into a pro- 
jecting process, which corresponds to an incision in the border of the orifice on either side. 
4. Opercular Muscles . — The motions of the operculum in the opening and closing of 
the orifice are effected by two pairs of muscles, occlusor and retractor, of which the 
former are the more important ; these muscles are sometimes inserted with the inter- 
vention of a long slender tendon and sometimes immediately. The points and mode of 
attachment should be carefully noticed. 
B. The other more important chitinous elements are the avicularian mandibles and 
vibracular set*. 
1. Mandibles . — These, like the opercula, exhibit very great variety of form, and not 
unfrequently several forms co-exist in one and the same species. But except in size, in 
which they often present greater differences than do the opercula, the characters of each 
particular type will be found remarkably constant. 
Like the avicularia of which they form part, the mandibles may be divided into those 
adapted for a prehensile purpose, and those which form merely a movable lid to the cup 
or receptacle upon which they are seated. 
In one division of the genus Cellepora (but only in those belonging to the southern 
hemisphere), in which the lower border of the orifice is straight and entire, the avicularian 
mandibles always present a slender process rising from the middle of the base, occasionally 
furnished with short hairs, to which I have applied the term columella. 
