CIRRIPEDIA. 495 



Sub-class III. Malacostraca. 



Division a. Hedriophthalmata. 



Order i. Amphipoda. 

 „ 2. Isopoda. 



Division b. Podophthalmata. 



Order i. Cumacea. 



,, 2. Stomatopoda. 



,, 3. Schizopoda. 



,, 4. De capo da. 



As regards the general distribution of the Crustacea in time, re- 

 mains of animals belonging to this class are abundant in the fossil- 

 iferous formations generally, from the Cambrian onwards. Nor are 

 these remains confined to purely marine formations, since many 

 Crustaceans are inhabitants of fresh or brackish waters, and the 

 remains of such are of common occurrence in lacustrine or estuarine 

 deposits. The most ancient group of the Crustaceans is that of the 

 Trilobites, which is represented early in the Cambrian period, and 

 continued to exist till the close of Palaeozoic time (Permian). 

 Nearly as old a group is that of the Phyllocarida, the latest repre- 

 sentatives of which appear in the Trias. The Eurypterids, again, 

 like the Trilobites, are exclusively Palaeozoic, the earliest forms of 

 this group appearing in the Ordovician period. Most of the other 

 orders of Crustaceans appear to have come into existence within the 

 limits of the Palaeozoic period, though some doubt may be felt as 

 to the systematic position and affinities of some of the ancient forms 

 which have been referred to certain of the higher orders of the class. 

 The Decapod Crustaceans, which represent the highest stage of 

 development in the class, have their beginnings in the Palaeozoic 

 period, but they do not attain a dominant position till the Mesozoic 

 deposits are far advanced, and they have attained their maximum 

 at the present day. 



Sub-class I. Axchoracephala. 



Order Cirripedia. 



The order of the Cirripedia includes a series of aberrant Crusta- 

 ceans, of which the most familiar forms, and the only ones which 

 occur in the fossil condition, are the Acorn-shells and their allies 

 (Balanidce and Verrucidiz) and the Barnacles (Lepadidce). The 

 essential feature of the order is that the larva has the form of 

 a free-swimming " nauplius," but the adult is fixed by means of its 

 modified a?ite?uice, and is enclosed in an integumentary sac, within 



