REPORT ON THE CRUSTACEA MACRTJRA. xxix 



is one also on the inner side, but it is wanting in this genus on the first or coxal 

 joint. 



The second joint or basis, the basocerite of Milne-Edwards, has on the outer and 

 lower angle a double-lobed calcified process corresponding with one developed on the 

 first or coxal joint, and which rotates against it, by the single lobe of the latter, which is 

 formed by a simple convolution of the hard wall, falling between the double lobe of the 

 basisal joint. On the inner side of the basisal joint the articulating process is also 

 developed, but there is no corresponding one on the coxa with which it can articulate. 

 Probably this is primarily due to the fact of the large projection of the sternal portion 

 of the first antennal somite precluding calcareous development in the inner walls of the 

 coxal joint of the second pair of antennae. The inner articulating process of the basisal 

 joint having no point of attachment has a free motion, and being pressed upwards, rests 

 upon the anterior portion of the projected sternum of the first antennal somite; the 

 attaching membranous tissue is consequently largely developed and overlies it also. On 

 the inner surface of this membranous fold, between it and the sternal portion of the first 



Fig. VIII. — Palinurus vulgaris. Basisal joint of second antenna, showing stridulating organ. 



antennal somite, just where it joins the concave surface of the hard wall of the antennae, 

 two small chitinous plates are developed ; one is comparatively large, ovate, and obliquely 

 striated with regularly corresponding fines, it is elastic in structure and opaline in appear- 

 ance ; the other is small, ovate, with a smooth surface, and amber coloured ; below these, 

 planted in a furrow, there is a line of thickly-set hairs. These structures form the 

 stridulating organ (PI. Xa. fig. c), and the joint instead of being articulated at both 

 extremities with the preceding as is usual in other forms, has the inner surface free and 

 capable of being played forwards and backwards over the smooth wall of the first antennal 

 somite, thus producing a sound that may be heard at a distance, even when produced 

 artificially after death. 



The fact that the common rock-lobster possesses the power of making a sound by 

 means of the antennae has long been known to our fishermen. It was mentioned 

 by Dr. Leach in his Malacostraca Podophthalma Britannica, but the sound-producing 

 structure was first described by Dr. Karl Mobius in 1867. * More recently it has been 



1 Archiv f. Naturgesch., Jahrg. xxxiii. p. 73, 1867. 



