INTRODUCTORY NOTE TO THE CRUSTACEA. 803 
In the Fourth type, there are six (or five) cephalothoracic annuli, as in the 
Entomostraca, with: which group they might be associated. But other 
peculiarities lead to a separation, and the species referred to are the 
Cirripedia. 
In the Fifth type, there are five (or four) cephalothoracic annuli, out of 
the whole normal number fourteen; in other words, the mouth never 
includes more than a single pair of maxillae with the mandibles. More- 
over, by the second law of degradation, all the jointed cephalothoracic 
appendages are wanting. These are the Rotatoria. 
The following table presents a view of the number of cephalic annuli in 
these Types, and also the mean size: 
Typical No, of cephalic Mean normal 
‘annua, Jength in lines. 
Type I. Decapoda or Podophthalmia, — Sub-type I. Brachyura, ; 9 24 (and breadth 24). 
Sub-type II. Macrura, 36 (and breadth 6). 
Type II. Tetradecapoda, ie wow eS Ce 6 
Type IIL. Entomostraca, a 1 
Type IV. Cirripedia, Ce 2?) 1 
‘Type V. Rotatoria, oe «© «© © eo 8 © 0 lw it 1-9 
See pp. 1406 and 1407 (loc. cit.), for observations on mean. size in 
the Entomostraca and Cirripedia, where an important principle is brought 
out, and where, also, some explanations are furnished which make the state- 
ment given above of the mean size, intelligible. — Ep. 
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