LIFE IN PRIMARY PERIOD 213 



The constant number of these appendages leads us to believe 

 that the Crustaceans may very well be descended from the 

 Trilobites. They must have diverged very early, however, 

 for in the Cambrian Epoch true Crustacea exist already, with 

 large membranous appendages serving both for swimming and 

 breathing, like those of our present Apas, which still resembles 

 the. Trilobite in some respects, or our Branchipus, Artemia, and 

 the Estheridse. 1 There are also Crustaceans related to Cypris, 

 commonly known as Water Lice, and frequently used in 

 pisciculture to feed the fry. These have received the name of 

 Ostracods, 2 because of their bivalve carapace resembling the 

 shell of the Lamellibranch Molluscs. These two orders were 

 thenceforth reinforced by the appearance of new genera, which 

 in some cases displaced the older genera and in others lived 

 alongside them. From the Silurian Period onwards the 

 Ostracods were replaced by the Cirripedes, represented to-day 

 by Balanidae or acorn-shells, shaped like sharp-edged 

 truncated pyramids, with which the rocks bristle at low-tide, 

 and Barnacles which suspend themselves by long peduncles to 

 floating wood or to the keels of ships. For a long time there 

 Mas considerable hesitation as to the place these organisms 

 should occupy; they are born in naupliiis form (p. 124), 

 a character common to all lower Crustaceans, and are later 

 transformed by the acquisition of a bivalve shell and six 

 pairs of bifurcated abdominal legs, into little organisms so 

 closely resembling Ostracods that they have been called 

 cypridian larvae. This stage implies a relationship not with 

 the present Ostracods, but with some still unknown pre- 

 Cambrian Ostracod. The cypridian larva attaches itself, by 

 means of a sucker in its antennae, to some solid submerged 

 body, and it is only then, while undergoing an important 

 change in attitude, that it finally acquires the characters of 

 a Cirripede. 



Myriapods 3 and Insects have also been discovered in the 

 Silurian strata. This is important. We have seen that 

 Myriapods are derived from the early entomostracan 

 Crustaceans, and Insects from the higher Crustaceans, or 

 Malacostraca. Hence the existence of Myriapods and Insects 



1 Hymnenocaris, Protocaris. 



2 Isoxys, Leperditia, Primitia. 



3 Archidesmus. 



