328 THE CRUSTACEA. §$ 278. 
usually concealed in special fossae. The tetragonal or hexagonal facets of 
the cornea are always very numerous; “ — behind each of them, is a con- 
ical, or prismatic lens, the round extremity of which is fitted into a trans- 
parent conical fossa, corresponding to a vitreous body; while the conical 
extremity of these bodies is received into a kind of calyx, formed by the 
filaments of the optic nerve. Each of these filaments, together with its 
calyx, is surrounded by pigment matter in a sheath-like manner.” * 
CHAPTER V. 
DIGESTIVE APPARATUS. 
§ 278. 
The opening of the digestive apparatus with the Crustacea is usually 
situated directly in front of the first pair of feet, which, as foot-jaws, grasp- 
ing or prehensile organs, are used for the seizing, the tasting, and the 
bearing to the mouth of food.” With many species, there are, as auxil- 
jary organs for this purpose, the oar-like, the post-abdominal, and branchial 
feet, the movements of which not only produce currents of water necessary 
for respiration, but also direct towards the mouth a great quantity of nutri- 
tive matter. 
Toe mouth is generally situated underneath and somewhat removed from 
the anterior border of the head. It is covered with a soft upper lip, 
~ 
26 The facets are tetragonal with Astacus, Ho- 
marus, Palinurus, Galatea, Scyllarus, Palae- 
mon, Pasiphaea, and Penaeus ; hexagonal with 
Scutigera, Squilla, Phyllosoma, Pagurus, Cali- 
, Mata, Campilius, Portunus, and Ilia ; 
see Milne Edwards, Hist. de Crust. I. p. 117, Pl. 
XII. and Will, Beitr. z. Anat. der zusammengeset. 
Augen, &c., p. 7. fig. 3, c. 
27 Will, loc. cit. p. 12, fig. 38,45 see also Suck- 
ow, loc. cit. Taf. X. fig. 19, 20 ; Soemmering, De 
‘oculor. sect. horizont. p. 75, Tab. III., and Milne 
Edwards, Hist d. Crust. Pl. XII. fig. 8 (Astacus). 
1 See above, § 268. : 
2These acts may be distinctly seen with the 
* [§ 277, end.] There is another form of eye 
observed by Dana (Report on Crust. loc. cit. p. 
1026) with Corycaeus and Sapphirina, and of so 
remarkable a character that I quote his description : 
“A pair of simple eyes, consisting of an internal 
prolate lens, situated at the extremity of a vermi- 
form mass of pigment, and of a large, oblate, lens- 
shaped cornea, The cornea is connected intimately 
with the exterior shell of the front or-the under 
-side of the head, and the two corneae are like spec- 
tacles adapted to the near sighted lenses within ; 
their size is extraordinary, being often one-third 
Phyllopoda, the Lophyropoda, and the Cirripedia. 
These last use principally their long, posterior, 
civrus-like feet, which they unroll and roll up 
alternately, maintaining regular currents in the 
water. During these movements, the three pairs 
of anterior and shorter feet seize, with much ad- 
dress, the particles of food borne against them by 
the current. Often the oar-like feet with the Daph- 
nioidae become dirty in this act, and are glued to- 
gether by particles of food which have been ejected 
from the mouth. But these animals easily relieve 
themselves by curving in front their spinous tail 
and combing out the oar-like feet, which are them- 
selves ciliated and bristled. 
of the greatest breadth of the body in Corycaeus. 
The lens and the cornea are often very distant from 
each other, being separated by a long clear space, 
The external surface of the cornea is spherical ; 
but the inner is conoideo-spherical, or parabolic. 
The texture is firm, and when dissected it breaks 
or cuts like a crystalline lens. The true lens is 
always prolate, with a regular contour, excepting 
behind, where it is partly penetrated by the pig- 
ment. The pigment is slender, vermiform, of a deep 
color, either red or blue, but at its anterior extremity 
usually lighter, and often orange or yellow." — Ep. 
