THE MANDIBLES 39 
sound. Just such a membrane Milne-Edwards considers 
to exist at the base of the antennz now under discussion, 
and in some of the Brachyura he and his colleague Audouin 
had investigated an inner apparatus capable of increasing 
the tension of the disk at the will of the animal, an 
arrangement which he compares with that of the auditory 
ossicles and the tympanic membrane of the human ear. 
It is only with reluctance that this description of a natural 
telephone can be relinquished. In some species, such as » 
the common rock lobster, Palinurus vulgaris, there is a 
stridulating apparatus in the basal joints of the second 
antenne, and it is obviously unlikely that a sound-pro- 
ducing organ should have been developed in an animal’s ear. 
4. The fourth or mandibular segment is of great im- 
portance, since from this, or from it in conjunction with 
the preceding segment, the carapace is developed. Its 
appendages also, the mandibles, yield in value to very 
few of the other organs. In form they vary extremely, 
but are for the most part of powerful structure. Their 
edges meet over the mouth-opening between the upper lip 
and the lower. The trunk of the mandible is frequently 
massive, with a projecting, finely denticulate, grinding 
surface called the molar tubercle, and a thick or thin dentate 
cutting edge, often having also a variety of spines between 
these two processes. It is not seldom surmounted by a 
narrow piece, commonly called its palp, which never in 
the Malacostraca consists of more than three joints. Very 
rarely, and only among the Entomostraca, one of the joints 
of the palp has an outgrowth supposed to represent the 
exopod. Since theoretically the exopod always arises 
from the second joint of an appendage, it is argued that 
the trunk of the mandible must represent the first joint. 
But to this it may be answered that the exceptional out- 
growth just mentioned is perhaps not an exopod, and that 
at any rate in the first antennz there is a similar out- 
growth from the third joint. In Huchceta glacialis, Hansen, 
and some other Entomostraca, the mandibular palp divides 
into two branches from its second joint. Seeing that the 
first joint of a crustacean appendage is very rarely of large 
