THE MANDIBLES 39 



sound. Just such a membrane Milne-Eclwards considers 

 to exist at the base of the antennae 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 

 antennae, 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 antennae there is a similar out- 

 growth from the third joint. In Euchceta 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 



