CRUSTACEANS. 619 
In the river- and sea-crays RosenTHat first described and 
ficured 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 antenna 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}. 
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. Fasricrus, and Minast and Scarpa, and others, after him, 
described and figured it in the river-cray (common cray-fish). At 
the base of the outermost antenne 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 antennz from the cerebral ganglion®. 
The compound eyes in crustaceans are formed on the same plan 
as in insects. BLAINVILLE found in them the vitreous humour, as 
did Jon. Mvetter after him in the eyes of insects ; and his descrip- 
tion of the eyes of Palinwrus agrees with that which we have given 
above of the compound eyes of insects*. 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, 
Avpourn, but which have not been made further known, this sense would seem to be 
much developed in crustaceans. Hist. nat des Crust. I. pp. 112, 113. 
1 See RosSENTHAL in Rew’s Archiv f. d. Physiol. X. 1811, 8. 433, 436, figs. 1-4. 
Comp. TREVIRANUS Biologie, VI. 8. 308, 309. This part is by Farre considered to be 
the auditory organ, Philos. Transact. 1843, p. 233 3; 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, I. 8. 255, strongly corroborate Farrn’s conclusions. | 
2 A. Scarpa, Anatomice disquisitiones de Auditu et Olfactu, Ticini, 1789, folio, 
pp. 2, 3, Tab. Iv. v.; comp. also E. H. WEBER, de Aure et Auditu hominis et animalium, 
Pars 1. Lipsiz, 1820, 4to. pp. 8, 9, Tab. I. figs. 5, 2. The membraneous tube situated 
in the pedicle belongs to a larger sac placed behind it, Branpt Mediz. Zool. 11. s. 64, 
Tab. XI. fig. 13, @, 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, Minne Epwarps Hist. nat. des Crust. 
I. p. 124; on the auditory organs of Crustacea, see T. H. Huxiey Zoolog. notes and 
observations in Ann. of nat. Hist. sec. Sér. Vol. vit. 1851, pp. 304—306, Pl. xiv. 
3 DucroTay DE BLAINVILLE, De Uorganisation des Animaux, 1. 1822, 8vo. pp. 
433, 434- 
