XX 
THE VOYAGE OF H.M.S. CHALLENGER. 
a. The prehensile mandibles, though differing greatly in form, have certain characters 
in common which distinguish them from the other kind. In shape they vary from 
a simple semicircle or triangle to a more or less elongated, blunt, or acutely-pointed 
spear- or sword-shaped, formidable, weapon. They are formed of a strong chitinous 
frame with a base, by each extremity of which the mandible is articulated to the 
sides of its cup or receptacle, in which are lodged the occlusor and retractor muscles, 
&c. In this class of mandibles the sides towards the apex or upper border of the 
lower face are usually, but not always, finely toothed, and the apex, in the spear- 
shaped type especially, is very frequently furnished with an acute incurved point or 
fang. The interior of the chitinous frame is occupied by a membrane in which is 
almost always a large rounded or elongated foramen, above which are inserted the 
occlusor muscles, which usually constitute a distinct pair, but in one family (Adeonese) 
appear to be conjoined into a single band. By this arrangement it would seem that 
power rather than speed in the occlusion of the prehensile avicularium is provided for. 
b. The retentive mandibles also present considerable variety of form, which may vary 
from a simple semicircle to all varieties of spatulate or duckbill-shape ; usually rounded 
or obtuse but sometimes more or less bluntly pointed at the apex. They are generally 
simple, but occasionally bifid or trifid. Unlike the prehensile mandibles, they are con- 
stituted for the most part of a thin membranous expansion, supported only at the 
base or partially at the sides and lower part by a chitinous frame, by which also they 
are articulated. In this class of mandibles also the foramen, if present, is always close 
to the base, very near to which the weak occlusor muscles are inserted ; we may con- 
clude therefore that the retentive mandible is shut down with a sudden snap, or with 
great rapidity and but little force. 
C. Besides the foregoing elements, which are almost universally present, others are 
met with which are limited to special groups ; for instance, in the Family Salicornariadse 
they consist of ; — 
1. A pair of slender curved rods imbedded in the ectocyst, one on each side of the 
orifice, and serving as points d’appui to the operculum ; these I have termed lateral 
trabeculae, but in some cases they are conjoined so as to form a complete or incomplete 
ring. 
2. A very delicate chitinous, apparently tubular, filament contained in the areolar 
ridges, and constituting an “ areolar network" continuous throughout each internode of 
the zoarium. (A somewhat similar network probably exists in some of the Selenariadse.) 
Very minute trabeculse, somewhat similar to those of the Salicornariadse, are found 
also in the genus Steganoporella, not on the sides but below the angles of the orifice. 
The only other chitinous elements to which reference need be made are — 
3. The so-termed radical tubes or radicells. 
4. The different forms of clasping or connecting tubular f laments. 
