chap, in.] ARTHROPODA. 65 



at the sides of the body there were a number of 

 appendages which were only incompletely ringed, but 

 the presence of which afforded evidence of metameric 

 segmentation. The mouth was near, though not 

 quite at, the anterior end of the body, and at its side 

 were a pair of slightly modified appendages ; the 

 anus was posterior and terminal. The excretory 

 organs were on the type of the Annulata, and were 

 arranged metamerically. Peripatus may form the 

 type of the Protracneata. 



In all the remaining Arthropoda, some of which 

 in all probability did not have a Peripatus-like an- 

 cestor, but have acquired a form similar to that of the 

 descendants of such an ancestor, owing primarily to 

 similar external conditions and similar necessities of 

 life (homoplasy, see page 12), the appendages are dis- 

 tinctly jointed, so that the separate parts can be 

 moved on one another ; the mouth is often some way 

 from the anterior end, and excretory organs of the 

 annulate type are never found. 



In the simpler forms the greater number of meta- 

 meres remain distinct, but in all divisions there is a 

 marked tendency for the metameres at the anterior end 

 to fuse into a head, and in some cases also into 

 a thoracic region. 



They are divisible into three great groups : A. 

 Crustacea, B. Arachnida, C. Tracheata. In 

 all three chitin is largely developed in the integument ; 

 and they are all, in addition, remarkable for the total 

 absence of those delicate protoplasmic processes which 

 we have learnt to know as cilia. 



A. The great majority of the Crustacea are 

 aquatic forms, and they either breathe the oxygen dis- 

 solved in the water in a vague manner (that is to say, 

 no special respiratory organs are developed, and the 

 exchange of gases is effected through the walls of the 

 body), or they are provided with outgrowths of the 



F 16 



