xviii THE VOYAGE OF H.M.S. CHALLENGER. 
2. As to function — 
(a) Prehensile — when the mandible, beak, and muscles are adapted for prehensile 
purposes. 
( b ) Retentive — when the mandible is thin, membranous, and weak, and adapted 
merely to serve as a lid to the cup or receptacle. 
3. As to position — 
(a) Vicarious — when they represent or replace an ordinary zooecium. . 
( b ) Adventitious — when either attached to some part or other of a zooecium or 
interspersed among the zocecia. 
B. Vibracula, of which two varieties may be distinguished — 
1. Simple — consisting of a basal cup, without a beak, to which the seta or flagellum is 
articulated, usually by a double joint, admitting of motion in only one plane. 
2. Compound — in which the seta is continuous with or articulated to a basal mandibular 
portion, and the cup or receptacle has a more or less distinct beak. 
VI. Chitinous Elements. — As will be seen in the Report, I have in several 
families largely resorted to the chitinous elements of the skeleton for diagnostic 
characters, for which purpose it is in many cases impossible to over-rate the value of 
these parts, as I have attempted partially to show in another place, 1 and further study 
has only convinced me that their importance extends far beyond the mere distinction of 
genera and species. But to render this manifest it would be necessary to embrace many 
forms besides those contained in the Challenger collection, and I am not as yet pre- 
pared to go fully into the subject, even were this the proper place. 
The chitinous elements here principally intended, are the so-termed opercula or oral 
valves and the chitinous parts of the avicularia and vibracula, that is to say, the mandibles 
and setce or flagella. 
A. Opercula. — Although there are one or two forms which must be ranged under the 
Cheilostomata, in which the existence of a distinct operculum or articulated lid to the 
orifice cannot be detected, such an appendage in some form or another may be regarded 
as an all but universal characteristic of the sub-order. 
The infinite variety in form and structure, and mode of muscular attachment of the 
opercula, is very remarkable, as is also the constancy of the characters presented by 
them in the different species, genera, and families. 
1. Form . — They are all more or less circular or semicircular in outline, with the lower 
border either straight or sinuated, or forming the segment of a smaller 'circle than the 
upper part, or by a further constriction produced in the middle into a narrower or 
1 On the Use to be made of the Chitinous Organs in the Cheilostomata in the Diagnosis of Species, &c., Journ. 
Linn. Soc. Land. (Zool.), vol. xv. p. 357, 1881. 
