Parker. — On the Structure of the Head in Palinurus. 301 



which the surface of the carapace is covered. As a result of this, the 

 ophthalmic sternum (ojrfi) is quite bare and fully visible in a view from 

 above (fig. 5), there being, further, no trace of "clasping processes" developed 

 from the antennulary sternum. 



Still more striking are the modifications of the antennulary sternum (fig. 

 2, 5, and 7, ant. st.) : it "forms a projecting vertical keel, with a rounded 

 anterior border, and slightly convex sides ; the anterior border is grooved 

 in the middle line, the groove being bounded on each side by a strong 

 smooth ridge (?•) ; each lateral surface is marked with a shallow groove (g), 

 and between the groove and the border, to which it is parallel, the surface 

 (s) is so smooth as to have the texture of polished ivory."* This keel, which 

 constitutes the fixed portion of the stridulating organ, is broad at its postero- 

 dorsal extremity, and thus interposes a wide space between the dorsal ends 

 of the basicerites, but ventrally it becomes much narrower, so that the basi- 

 cerites, being applied closely to its lateral surfaces, are very closely approxi- 

 mated at its antero-ventral extremity (fig. 5). For this reason the bases of 

 the antennules are concealed, in a view from above, by those of the an- 

 tenna, as in P. edwardsii. 



The articulation of the basicerite with its socket presents the noteworthy 

 peculiarity of the absence of the dorso-internal hinge. From the spot 

 where this hinge occurs in P. edwardsii, a strip of uncalcified chitin extends 

 distalwards, and is so modified as to form the movable portion of the stri- 

 dulating organ. The upper edge of the strip of chitin is produced into a 

 large outstanding flap (figs. 5 and 8, fl.), immediately beneath which " the 

 chitin becomes thickened and takes on the form of an oval area or pad (p) 

 marked by a number of fine parallel ridges. In relation with the inferior 

 edge of this pad is a small calcified tubercle (t). When the antenna is in 

 place the flap works over the ridge (r) on the corresponding side of the 

 anterior border of the antennulary sternum, the tubercle fits into the 

 groove (g) on its lateral surface and the ridged pad is closely applied to the 

 smooth space (s) between the groove and the border. Under these circum- 

 stances, when the antenna is moved upwards, the friction of the pad against 

 the smooth surface produces a loud and grating noise, the principle being 

 similar to that of the sound produced by the friction of india-rubber against 

 paper. The apparatus can at any time be thrown out of gear and the 

 antenna move noiselessly by slightly abducting the latter."! 



In consequence of the absence of the upper hinge the movement of the 

 antenna is not, as in P. edwardsii, confined to one plane. I am disposed to 

 think that the additional abductor muscle, described above in that species, 

 has, in P. vulgaris, the special function assigned to it of abducting the 

 antenna when it is to be moved noiselessly. 



* Proc. Zool. Soc, 1878, p. 292. t Proc. Zool Soc, 1878, p. 292. 



