CRUSTACEANS. 619 



In the river- and sea-crays KOSENTHAL first described and 

 figured an organ as that of smell, which afterwards was also found 

 in some other decapods, but is wanting in most species of this order, 

 as well as in the remaining crustaceans. At the base of the middle 

 or innermost antennas he found a triangular opening beset with 

 hairs, which is the entrance of a triangular cavity lined with a soft 

 membrane, in which nervous branches are distributed 1 . 



Just as little is known of the organ of hearing in most animals 

 of this class. In the ten-footed crustaceans it was discovered by 

 J. C. FABRICT US, and MINASI and SCARPA, and others, after him, 

 described and figured it in the river-cray (common cray-fish). At 

 the base of the outermost antennae is a very hard wart-like ex- 

 crescence, at whose point is a round opening covered by a tense and 

 very elastic membrane. Behind this membrane is a vesicle filled 

 with fluid, on the walls of which a nerve is distributed that arises 

 with the nerve of the external antennae from the cerebral ganglion 2 . 



The compound eyes in crustaceans are formed on the same plan 

 as in insects. BLAINVILLE found in them the vitreous humour, as 

 did JOH. MUELLER after him in the eyes of insects ; and his descrip- 

 tion of the eyes of Palinurus agrees with that which we have given 

 above of the compound eyes of insects 3 . In many crustaceans, for 

 instance in the crays, the facettes of the cornea are not hexangular, 

 as in insects, but quadrangular ; in some other ten-footed crustaceans, 



AUDOUIN, but which have not been made further known, this sense would seem to be 

 much developed in crustaceans. Hist, not des Crust. I. pp. 112, 113. 



1 See ROSENTHAL in REIL'S Archivf. d. Physiol. x. 1811, s. 433, 436, figs. 14. 

 Comp. TREVIRANUS Biologie, vi. s. 308, 309. This part is by FARRE considered to be 

 the auditory organ, Philos. Transact. 1843, p. 233 ; comp. however hereon ERICHSON'S 

 report in his Archiv f. Naturg. 1844, s. 336, 337, who participates as little in that 

 opinion as I, for my part, am able. [Additional investigations by LEUCKART, Archiv 

 f. Naturgesch. 1853, * s - 2 55> strongly corroborate FARRE'S conclusions.] 



2 A. SCARPA, Anatomicce disquisitiones de Auditu et Olfactu, Ticini, 1789, folio, 

 pp. i, 3, Tab. iv. v. ; comp. also E. H. WEBER, deAure et Auditu kominis et animalium, 

 Pars I. Lipsise, 1820, 4to. pp. 8, 9, Tab. I. figs, i, 2. The membraneous tube situated 

 in the pedicle belongs to a larger sac placed behind it, BRANDT Mediz. Zool. II. s. 64, 

 Tab. XT. fig. 13, a, a. In the short-tailed decapods this membrane covering the entrance 

 to the auditory sac is represented as a moveable calcareous plate. See on this little 

 plate and its peculiar arrangement in Maia, MILNE EDWARDS Hist. not. des Crust. 

 I. p. 124 ; on the auditory organs of Crustacea, see T. H. HUXLEY Zoolog. notes and 

 observations in Ann. of not. Hist. sec. Se>. Vol. vn. 1851, pp. 304 306, PI. xiv. 



3 DUCROTAY DE BLAINVILLE, De V organisation des Animaux, I. 1822, 8vo. pp. 

 433, 434- 



