Fifth Section. 



Arthropoda. 



General Review of the Group. 



§ 182. 



In this division of the Animal Kingdom the body consists of a 

 number of metameres, and this number is generally definite for the 

 various groups. As a rule the metameres are not all differentiated 

 to the same extent ; this is not implied merely by variations in 

 external form and size, but also in the characters of the internal 

 organs of the body. A number of metameres may be united to 

 form larger segments, in which the separate metameres lose their 

 individuality. These larger segments sometimes retain indications 

 of this kind of composition ; but these indications are sometimes 

 lost, or only apparent in the early stages of development. This 

 state of things results in a desegmentation of the body. 



The movable appendages of the body form another characteristic 

 of the whole group ; these appendages are almost always segmented. 

 In this point therefore, as in the metameric character, they have 

 some resemblance to the Annulata among the Vermes. We do not 

 know what are the forms that ally the two groups, and it is not cer- 

 tain that the two chief groups of the Arthropoda form a common 

 phylum. There are many reasons for regarding the Branchiata and 

 Tracheata as being distinct stem-forms. As in the Annulata the 

 nervous system consists of an oesophageal ring connected with a 

 ventral ganglionic chain ; while the central organ of the circulatory 

 apparatus is here, also, dorsal in position. In the Vermes organs are 

 repeated in each segment, but in the Arthropoda they are common to 

 the whole of the body ; and even when the metameres are similar 

 externally, the internal organisation often gives signs of the more 

 confined dominion over the whole body now possessed by meta- 



