CLASS IV. CRUSTACEA: ORDER 10. PARASITA. 



613 



ORDER 7. PHYLLOPODA. 



This order includes tlic Ax)odidce, in wliicli the body is protected by a carapace which takes 

 the form of a bivalve shell, and also the Branchipodidce, which are without carapace. One 

 species, Branchipus stagnalis, is often found in cart-ruts. This also seems to be the place for 

 a family of fossil Crustacea, the well-known Trilobites, Trilobita, of which vast 

 numbers occur in some of the earlier strata of the earth's crust. Their general 

 form is shown in the annexed figure of the Calymene Blumenbachii ; they pos- 

 sessed well-formed, compound, faceted eyes, which are frequently well preserved 

 in the fossil state. The body is usually divided into three regions, of which the 

 first and last are commonly in the form of semicircular plates, while the middle 

 portion exhibits distinct segmentation, and by its flexibility enabled the animal 

 to double itself up in the manner of the common wood-louse. These animals are 

 now quite extinct, although during the period of the deposition of the ancient 

 strata in which their remains are found, they were almost the only representa- 

 THE CALTMKNE. tlvcs of tlic class Crustacca. 



ORDER 8. OSTRACODA. 



In this order, composed of animals generally of very minute size, the body, which strongly 

 resembles that of the Copepoda, is always inclosed in a little bivalve shell, the feet and antennae 

 being protruded between the lower edges of the valves. These little shells so closely resemble 

 those of minute bivalve mollusca, that those of some of the larger species have actually been de- 

 scribed by conchologists as the coverings of animals belonging to that class. The antennae are 

 often curiously branched, and the hinder extremity is usually produced into a sort of tail, which 

 is seen in constant action when the animal is in motion. 



This order forms two families — the Cyprklce, in which the body is entirely inclosed within 

 the shell, of which the genus Cypris is an example, and the DaphniadoB, in which the head is 

 protruded beyond the shell. In the Polyphemus, belonging to the latter, the head, which is 

 large, is almost entirely occupied by an enormous eye, giving the creature a most singular ap- 

 pearance. 



ORDER 9. COPEPODA. 



These are minute animals, with the body divided into segments. They appear to possess no 



distinct respiratory organs, and the ova are carried in sac-like 

 organs attached to the abdomen of the mother. They occur in 

 countless swarms in all waters, whether salt or fresh, and, minute 

 as they are, one species is said to constitute the principal food 

 of the Antarctic whale. The best-known form is the genus Cy- 

 clops, specimens of which may be found in every stagnant pool ; 

 it is the type of the family Cyclopidce, characterized by the pos- 

 session of a single eye. The species Cyclops navicularis is found in stagnant fresh water in the 

 Southern States. A closely-allied species is found in Lake Ontario. 



THE CYCLOPS. 



ORDER 10. PARASITA. 



This order is composed of numerous small animals, which, in their young state, are furnished 

 with distinct jointed limbs, antenna?, and eyes, organs which cither disappear completely or be- 

 come greatly modified as the animals approach maturity, when they attach themselves to fishes 

 or other aquatic animals, and pass the remainder of their existence as parasites. Tliere are sev- 

 eral families : one species, the Argulus foUaceus, is very common upon various European fresh- 

 water fishes. The A. catostomi is found on the gill-covers of a species of sucker in Connecticut, 

 and the A. alosce, on the gills of the alewife in Massachusetts. 



