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, togeth^er 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- 

 iary 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.*-' 



The mouth is generally situated underneath and somewhat removed from 

 the anterior border of the head. It is covered with a soft upper lip, 



2fi The facets are tetragonal with Astacu.i, Ho- 

 marus, Palinurus, Galathea, Sr.yllarus, Palac- 

 mon, Pasiphaea, and Penaeus ; hexagonal with 

 Scutigera^ Squilla, Phyllosoma, Pagurus, Cali- 

 aiiassa, Maia, Campilius, Portiaius, and Ilia ; 

 see Milne Edwards, Hist, de Crust. I. p. 117, PI. 

 XII. and fVill, Beitr. z. Anat. der zusammeugeset. 

 Augen, &c., p. 7. fig. 3, c. 



27 IVill, loc. cit. p. 12, fig. 3, 4 ; see also Suck- 

 ow, loc. cit. Taf. X. fig. 19, 20 ; Soemmerins, De 

 ooulor. sect, horizont. p. 75, Tab. III., and Milne 

 Edwards, Hist d. Crust. PI. XII. fig. 8 {Astacus). 



1 See above, § 268. 



2 These acts may be distinctly seen with the 



Phyllopoda, the Lophyropoda, and the Cirripedia. 

 These last use principally their long, posterior, 

 cirrus-like feet, which they um-oU 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 ijarticles of food borne against tliem by 

 the current. Often the oar-like feet with the Daph- 

 nio'idae 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. 



* [ § 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 



of the greatest breadth of the body in Cnrycaeus. 

 The lens and the cornea are often very distant from 

 each other, bemg 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 crystalhne 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.'" — Ed. 



