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TEXT-BOOK OF ENTOMOLOGY 



FIG. 2. Restored section of Calymene : 

 C, carapace: en, endopodite ; en', exopodite ; 

 with the gills on the epipodal or respiratory part 

 of the appendage. After Walcott. 



abdominal), showing that they are a much more primitive type, and 

 nearer to the annelids than any other Arthropoda. Their gills, as 



shown by the researches of Wal- 

 cott and of Beecher, were at- 

 tached to nearly if not every 

 pair of limbs behind the antennae 

 (Figs. 1, 2). The fact that in 

 Trilobita the first pair of limbs 

 is antenniform does not prove 

 that they are Crustacea, since 

 Eurypterus has a similar pair 

 of appendages. 



The limbs in trilobites, as well 

 as the abdominal ones of merostomes, and all those of Crustacea, 

 except the first antennae, are biramose, consisting of an outer (exopo- 

 dite) and an inner division (endopodite). In this respect the ter- 

 restrial air-breathing tracheate forms, Arachnida, Myriopoda, and 

 Insecta, differ from the branchiate forms, as their legs are single or 

 undivided, being adapted for supporting the body during locomotion 

 upon the solid earth. It is to be observed that when, as in Limulus, 

 the body is supported by cephalic ambulatory limbs, they are single, 

 while the abdominal limbs, used as they are in swimming, are 

 biramose, much as in Crustacea. 



The Arachnida. The scorpions and spiders are much less closely 

 allied to the myriopods and insects than formerly supposed. Their 

 embryology shows that they have descended from forms related to 

 Limulus, possibly having had an origin in common with that animal, 

 or having, as some authors claim, directly diverged from some 

 primitive eurypteroid merostome. But they differ in essential 

 respects, and not only in the nature and grouping of their appen- 

 dages; the first pair instead of antenniform being like mandibles, 

 and the second pair like the maxillae, with the palps, of insects, the 

 four succeeding segments (thoracic) bearing each a pair of legs. 

 They also have a brain quite unlike that of Limulus, the nervous 

 cord behind the brain, however, being somewhat similar, though 

 that of Limulus differs in being enveloped by an arterial coat. 

 Arachnida respire by tracheae, besides book-lungs, which, however, 

 are possibly derivatives of the book-gills of Limulus, while they 

 perform the office of excretion by means of the malpighian tubes, 

 and like Limulus possess two large digestive glands ("liver"). 

 Their embryos have, on at least six abdominal segments, rudiments 

 of limbs, three pairs of which form the spinnerets, showing their 



