INTRODUCTORY NOTE TO THE CRUSTACEA. 303 



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 



Type I. Decapoda or Podophthalmia. — Sub-type I. Brachyura, ^24 (and breadth 24), 

 Sub-type II. Macrura, J 36 (and breadth 6). 



Type n. Tetradecapoda, 7 6 



Type III. Kntomostraca, 6-5 1 



Type IV. Cirripedia, ..•« 6-5 1 



Type V. Rotatoria, 5-4 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. — Ed. 



