SECT. II THE OUTER BODY FORM 19 



probably to the fact that the number of rudimentary 

 limbs actually does vary. And even if it should be 

 proved that the same species always possesses the 

 same number of segments developed and rudimentary, 

 the different species of Apus and Lepidurus are 

 marked by decided differences . in the number of 

 segments. Whereas the more specialised Crustacea 

 (the Malacostraca) have either the constant number 

 of twenty or twenty-one segments, the number in the 

 Apodidae varies between thirty-five and sixty-five. 

 In the Entomostraca the number varies, but never 

 reaches even the lowest number in the Apodidae. 

 That the number in the Apodidae is visibly decreasing 

 follows from our explanation of their morphology. 

 The fact that the posterior segments remain fixed, 

 in a larval and undeveloped condition, shows that 

 they are gradually being dispensed with. On this 

 ground alone, then, the Apodidae deserve to occupy 

 the place, half way between the Annelids and the 

 Crustacea, which we claim for them. 



Many of the segments, as already seen, are so 

 rudimentary as to be useless, i.e. as movable segments, 

 so that three, four, or even six combine to form one 

 body ring. In the Trilobites we shall find that the 

 posterior rudimentary segments, which were for the 

 same reason immovable upon one another, form 

 together, in many genera at least, a solid tail plate, 

 the pygidium (cf. Fig. 49, p. 220). 



It is a characteristic of the Crustacea that no limbs 

 develop on the anal segment. In the Apodidae, this 

 segment is already fully developed ; they are the 



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