BIONOiAIIC CHARACTERS OF CRUSTACEA 1025 



objects, known as conodonts, were formerly regarded as the teeth 

 of mixinoid fishes or as radulse of gastropods. 



VIII. Arthropoda. 

 Crustacea. 



Trilobitcr. The trilobites are extinct Palaeozoic Crustacea of an 

 undoubted marine habitat, probably able to swim as well as crawl, 

 and so belonging at one time to the nekton, at another to the vagrant 

 benthos. Whether or not a mero-planktonic larva existed is not 

 known, but it might be assumed from the wide distribution of some 

 species. Since trilobites cast off their exoskeletons, as does Limulus, 

 some of these may have floated a considerable distance, coming to 

 lodge where trilobites never lived. It is certain that from the 

 number of fossil trilobites we cannot judge the number of individ- 

 uals existing at a given place, since a number of specimens may rep- 

 resent the cast-oft' exoskeletons of one individual. Trilobites, like 

 many modern Crustacea, probably turned on their backs while sink- 

 ing to the sea-floor, this accounting for the frequent overturned 

 position in which their remains are found. 



PJiyllopoda; Cope pod a. These belong largely to the plankton, 

 the phyllopods occurring mostly in fresh water, the copepods having 

 fresh water (Cyclops) and marine representatives. The copepods 

 further comprise commensal forms, which live in the branchial 

 cavity of Ascidians ( Notodelphus) or on the carp (Argulus) ; and 

 a large number of parasitic types. Some of them, however, are 

 only occasionally or temporarily parasitic. Some phyllopods have 

 a bivalve shell (Estheria, etc.), which is frequently preserved in 

 the finer fresh water sediments of continental formations. A plank- 

 tonic nauplius larva occurs. The eggs of some phyllopods have 

 the power to withstand desiccation for years. In fact, the eggs 

 of Apus do not develop unless they have been subjected to desicca- 

 tion for some time (Semper). This accounts for the periodic 

 reappearance of these organisms in the temporary water bodies 

 of desert regions. The occurrence of such types (Estheria) in 

 otherwise unfossiliferous deposits thus indicates that these deposits 

 may be of desert origin. 



Ostracoda. The ostracods are marine or fresh-water, planktonic 

 or vagrant benthonic Crustacea whose imperfectly segmented body 

 is enclosed in a bivalve shell. The majority of the marine forms 

 are holo-planktonic, living in shallow water or in moderate depths. 



