242 ANNULATA. 
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 consolidation of the segments, and 
in the gradual adoption of embryonic development. 
There are two sub-classes—(1) Entomostraca and (2) 
Malacostraca. 
Susp-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 maxille, 
and there is never a gastric mill. The Lxtomostraca 
develop by a free-swimming xaupiius larva. 
Fig. 158.—THE LIFE-HISTORY OF CIRRIPEDIA. 
1. Nauplius larva of Balanus. Ventral 2. A rather later larval stage of 
view. Note three pairs of swimming Chthamalus. 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 (antennz) 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. Apus is a large phyl- 
lopod with a head-shield covering most of the body. The 
