Characteristics of the Arth7'opoda. cv 



correspondence with the number of the segments of the body, which 

 resembles that manifested by the multiple respiratory organs of 

 many Annvdata. But the circulatory, depuratory, and reproductive 

 organs are never found to be thus multiplied, such segmentation 

 as they may exhibit being ordinarily limited to one, and that the 

 abdominal region of the body. The digestive tract takes ordinarily 

 a very direct antero-posterior course, rarely presenting any lateral 

 diverticula except in Arachnida, or any convolutions, except in 

 adult Insecta, and some Cladocera. This latter Crustacean family 

 has the anus placed dorsally, and some way anteriorly to the termi- 

 nation of the body segments^ and furnishes an exception to the 

 general rule, that the intestine in Arthropoda ends in the last 

 segment of the body. Except in the larvae of Hymenoptera and 

 of Mtjrmeleon, the intestine is always proctuchous. But in the 

 suctorial Oirripedia, and in the ' complementary males ' of the other 

 families of that order, as also in a suctorial Entomostracan, Mon- 

 strilla Banae, the digestive tract is wanting altogether, and the 

 mouth when, as in the last case, present, leads directly into the 

 general cavity of the body. A heart is very usually present, 

 underlying the dorsal elements of the abdominal segments, and 

 ending in an aorta in the thoracic region. It is usually vasiform 

 and segmented, lateral apertures at the anterior end of each seg- 

 ment serving as venous inlets for the blood filling the pericardial 

 sinus in which the heart is suspended by means of the elastic alae 

 cordis. The circulation is always more or less extensively lacunar ; 

 even arteries may be wanting. 



Accordingly, as the respiration is aquatic or aerial, the Arthro- 

 poda are divisible into two great groups^ one of which is con- 

 stituted by the Crustacea, in which respiration is branchial, or in 

 the absence of branchiae, carried on in water by the general sur- 

 face of the body ; and the other l)y the other three classes, Ara- 

 chnida, Myriopoda, and Insecta, in which respiration is effected by 

 the admission of air into the interior of the body by tracheae, 

 or some modification of those organs. The air-breathing Arthro- 

 poda agree with each other, and differ from the Crustacea in the 

 following points : — they never have two pairs of antennae ; and, 

 with the exception of certain Ephemeridae, Strepsipiera, and Dlptera 

 amongst Insecta, and certain Epeirae amongst Arachnida, their 

 eyes are never pedunculate ; their mandibles are not palpate ; one 



