STRUCTUEE AND CLASSIFICATION OF THE AETHEOPODA. 569 



four or more additional following somites may be turned 

 upwards towards the mouth and assist in the taking of food. 



The more primitive forms (Entomostraca) are anomomeris- 

 tic, presenting great variety as to number of somitesj form of 

 appendages, and tagmatic grouping; the higher forms (Mala- 

 costraca) are uomomeristic, showing in front of the telsou 

 twenty somites, of which the six hinder carry swimmerets, 

 and the five next in front ambulatory limbs. The genital 

 apertures are neither far forward nor far backward in the 

 series of somites, e. g. on the fourteenth post-oral in Apus, on 

 the ninth post-oral in female Astacus and in Cyclops. 



With rare exceptions, branchial plates are developed either 

 by modification of a ramus of the limbs or as processes on a 

 ramus, or upon the sides of the body. No tracheate Crus- 

 tacea are known, but some terrestrial Isopoda develop 

 pulmonary in-sinkings of the integument. A characteristic 

 comparable in value to that presented by the pygidial shield 

 of Arachuida is the frequent development of a pair of long- 

 appendages by the penultimate somite, which, with the telson, 

 form a trifid, or when that is small a bifid termination to 

 the body. 



The lateral eyes of Crustacea are polymeniscous, with highly 

 specialised retinulas like those of Hexapoda, and unlike the 

 simpler compound lateral eyes of lower Arachnida. Mono- 

 meniscous eyes are rarely present, and when present single, 

 minute, and central in position. 



Note. — The Crustacea exhibit a longer and more complete 

 series of forms than any other class of Arthropoda, and may 

 be regarded as preserving the most completely represented 

 line of descent. 



Class 4. — Chilopoda, 



Head triprosthomerous ^ and tetartognathous. The tAVO 

 somites following the mandibular or first post-oral or buccal 



^ Embrjological evidence of this is still wauling. In llie other classes of 

 Arthropoda we have more or less complete embryological evidence on the 

 subject. It appears from observation of the embryo that whilst the first 

 VOL. 47, TAKT 4. NEW SERIES. 



