430 THE ARTHROPODA. 



logous, for they are not formed of the same number of segments 

 in all cases, nor are the same segments in all cases included in 

 the similarly named body-region. The absence of a definite rule 

 can be seen even in the Crustacea, in which the incorporation of 

 thoracic limbs with the mouth-parts varies greatly in the different 

 sub-divisions. Although we thus see that, in the great divisions 

 of the Arthropoda also, the consecutive segments develop hetero- 

 morphously, we shall still be inclined to explain this fact by the 

 requirements of the different functions, and shall not homologise 

 the regions bearing similarly formed appendages. It will thus be 

 quite possible to homologise the thoracic and abdominal appendages 

 of one division with the cephalic and thoracic limbs of another, as 

 in the table. That we are justified in such a course is shown, not 

 only by the condition of the Crustacea already mentioned, but also 

 of those Arthropods which we consider as the more highly developed, 

 such as the Insecta. In the Hymenoptera, for instance, the first 

 abdominal segment may join the thorax (segment mediaire),* and 

 be so marked off from the abdomen as to appear much more like 

 a thoracic than an abdominal segment. An omission, or rather a 

 complete degeneration of single segments, such as must be assumed 

 in the cases of the first antennae in the Arachnida and of the second 

 antennae in the Myriopoda and Insecta (see the table), seems to be 

 an exceptional occurrence. We are here leaving out of account the 

 reductions undergone in many cases by the Arthropodan body 

 (e.g., in certain Crustacea, Arachnida, Pentastomum, and also many 

 Insecta), and which often lead to a complete degeneration of the 

 segmentation in different regions of the body or in the whole body, 

 indeed, in the last case may even result in the disappearance of 

 the division into body-regions. Such reductions may lead to the loss, 

 not only of the segmentation, but also of the limbs (Pentastomum), 

 a principal character of the Arthropoda being thus obliterated, but 

 even in this case the development of larvae provided with extremities 

 testifies to the Arthropodan nature of these forms. 



* Such a significance cannot any longer be ascribed to the segment mediaire 

 of the Diptera (Bkauer, Silz. Akad. TViss. Wien, Bd. lxxxv., Abtb. i., 1S82), 

 but there is evidently a close connection between the thorax and the abdomen 

 in this older also. 



