450 



Div. 3. ARTICUL ATA.— ARACHNID A. 



Class 2. 



In Asaphus, Brong., the ocular tubercles appear to exhibit a covenng, or are ^anular ; the tail-piece ter- 

 ininating the body, is less elongated than in Calyinene, and nearly semicircular, or in the shape of a short triangle.* 



In Ogt/gia, Brong., the shield is longer than broad, mth the 

 posterior angles produced into a spine. The ocular promi- 

 nences exhibit neither covering nor granulations. The body 

 is elliptic. 



These eminences, having the appearance of eyes, either do 

 not exist, or are not distinctly to be seen, in the genus Para- 

 doxides, Brong. The segments, or at least the majority ot 

 them, extend laterally beyond the body, and are disengaged 

 at their extremity on the sides. 



Such are the characters of the five genera established by 

 M. Alex. BrongTiiart, and which may be arranged into three 

 groups : 1, the Reniformes (genus Agnostus) ; 2, theContrac- 

 tiles (g. Calymene) ; 3, the Extensi (g. Asaphus, Ogygia, and 

 Paradoxides). We refer for a knowledge of the species and 

 their respective strata, to the work of the above-mentioned 

 celebrated naturalist, who has associated with him, in respect to the fossil Cnistacea, M. Desmarest, so often cited 

 by us in our accounts of fossil and recent Crustacea. Other savans have proposed other genera amongst the Trilo- 

 bites ; but being confined to the most general considerations, I can only cite those which appear in the best 

 work yet published on these singular fossils. 



Fig. 27. — A, Asaphuu expansus. b. The same rolled up. 



THE SECOND CLASS OF ARTICULATED ANIMALS FURNISHED WITH 



ARTICULATED LEGS,— 



ARACHNIDA,— 



Is, like the Crustacea, [composed of species] destitute of wings, and which are in a 

 manner not liable to change their form, not undergoing metamorphosis, but simple 

 sheddings of the outer covering of the body. Their sexual organs are placed at a 

 distance from the posterior extremity of the body, being (except in some males) at 

 the base of the venter. But they differ from these animals as weU as from the true 

 insects in many respects. As in the latter, the surface of their bodies exhibits orifices 

 or transverse slits, named stigmata (but which it would be better to name Pneumo- 

 stomes, — mouth for the air, — or spiracles, that is, respiratory orifices), serving for the 

 entry of the air, but being few in number, (eight at most, generally only two), and 

 situated only on the under side of the abdomen. Respiration is effected either by 

 means of aerial branchiae, serving as lungs and inclosed in bags, to which these 

 spiracles form the entry, or by means of radiating tracheae. The organs of sight con- 

 sist only of minute simple oceUi, grouped in different positions when there is a 

 number of them. The head, generally united to the thorax, merely exhibits at 

 the place of the antenuce two articulated pieces, hke small didactyle or monodactyle 

 claws, which have been injudiciously compared to the mandibles of insects, and so 

 named ; but they move in a direction opposed to the motion of mandibles, or up 

 and down, assisting, nevertheless, in eating, and replaced, in those Arachnida which 

 have the mouth formed into a siphon or sucker, by two pointed plates, used as 

 lancets. t A sort of lower lip (labium, Fab.), or rather tongue, (languette), formed 



* In Ataphut^ Brongniart, described and figured b^ M. K. Dealong* 

 champs, the posterior angles of the shield, instead of being directed 

 backwards, as in the other species, are recurved. 



+ CheliceriE, or anteniial claws, for such they are eridently, as 

 proved by a comparison of these organs with the intemieiliate au. 



teiinoD i)f various Crustacea, especially those of the order PiecilODCda. 

 Hence it is not quite correct to say that the Arachnida are destitute 

 of antciniK, a negative character, by which thev have been defined 

 Dj preceding authors. 



