242 ANNUL AT A. 



If we trace the class from the lowest to the highest, we 

 can notice a general advance in size and complexity of the 

 body, in reduction and consoHdation of the segments, and 

 in the gradual adoption of embryonic development. 



There are two sub-classes — (i) Entomostraca and (2) 

 Malacostraca. 



Sub-Class I. — Entomostraca. 



These are nearly all small and simple Crustacea. There 

 is great variety in the number of the segments. The excre 

 tory organ (shell-gland) is situated on the second maxillae, 

 and there is never a gastric mill. The E?itomostraca 

 develop by a free-swimming nauplius larva. 



The Life-History of Cirripedia. 



I. Nauplius larva of ^«/at»«5. Ventral 2. A rather later larval stage oi 



view. Note three pairs of swimming Chthamahis. The posterior region is 

 appendages, the last two being biramous elongating, 

 and the median simple eye. 



3. Cypris larva of Lepas. Just fixed by 

 its anterior end (antennae) to a piece of 

 wood. Note the six pairs of biramous 

 appendages and the enveloping shield. 



The Phyllopoda have foliaceous or leaf-like appendages. 

 Some are small and are known as water-fleas. Daphnia 

 is a very common freshwater type. Apiis is a large phyl- 

 lopod with a head-shield covering most of the body. The 



