620 CLASS X. 



in the Stomapods, &c., they are, however, hexangular. In Limulus 

 they are also hexangular, not very regular, and at the outside but 

 little raised. In some crustaceans the eyes are covered with an 

 undivided smooth cornea, as in Apus; they are compound eyes 

 with a cornea without facettes 1 . They make as it were the tran- 

 sition to the clustered eyes, placed in two groups at the side of the 

 head of the Isopods, as Oniscus, Idotea, &c. 



The passive organs of motion of crustaceans are the hard cover- 

 ings of the body and of the limbs, the dermal skeleton. There are 

 also frequently productions of this covering penetrating inwards, 

 horny or hard calcareous projections to which the muscles are 

 attached. In Limulus an elongated, rectangular tendinous plate, 

 somewhat excavated on the upper surface, is situated in the inside 

 of the cephalothorax, just as in spiders, in which, at the fore part, 

 are two cylindrical processes that pass into tendons. This part is 

 moved by many muscles, and, with its movements, those of the feet 

 are also connected. The legs of decapods have at every joint a 

 flexor and extensor muscle; those of the first pair or of the so-called 

 claws, in accordance with the greater strength of this pair of limbs, 

 are the most developed. The muscles of the tail in the crays 

 (lobsters, cray-fish, &c.) are divided into two layers; the layer 

 which is situated on the ventral surface, that of the flexor muscles, 

 is more composite and more powerfully developed than the layer 

 which lies towards the back 2 . 



In art-instincts the crustaceans appear to stand below most 

 insects and arachnids. In them the vegetative or organic life is 

 more developed than the animal. 



The geographic distribution of crustaceans has not been hitherto 

 sufficiently investigated, although LATREILLE, and after him espe- 



1 See JOH. MUELLER in MECKEL'S Archiv, 1829, s. 54 59, Tab. in. fig. 15; in Gam- 

 tnarus pulex, ibid. figs. 16, 17; in JBranckipus, H. BURMEISTER in MUELLER'S A rcliiv, 

 1833, s. 529 534, s. 613, Tab. xin. figs, i 4. The conical transparent bodies of the 

 separate divisions of the eye are either attached immediately to the cornea or have 

 lenses placed before them, which lie under the cornea. See on these peculiarities 

 MUELLER Handbuch der Physiologic, II. s. 309. The eyes of Limulus do not however 

 belong to the division in which MUELLER places them. The organs of vision in Crus- 

 taceans are largely treated of in E. WAGNER Lehrb. der vergl. Anat., 2te Auflage, 

 liter Theil, bearbeitet von DR H. FREY u. DR R. LEUCKART, 1847, s - 202206. 



3 The tail-muscles of the river-cray are described and figured by CUVIER, Lcfons 

 (TAnat. comp. I. pp. 423 426, v. PI. xrv. 



