286 MR. H. M. BERNARD OK THE EELATIOlSr 



ABimal Kingdom. We have already elsewhere* endeavoured to 

 explain by it the comparatively low state of organizabion and the 

 diminutive size of many of the smaller forms of modern Crustacea. 

 Indeed, we might institute many very interesting parallels between 

 the smaller Crustacea and the Acaridse ; for instance, we have in 

 the Ostracoda the abdomen with very few segments developed, and 

 only occasionally a heart, the heart being, in all probability, the 

 anterior end of the long dorsal vessels of their more developed 

 ancestors — primitive Apodidas or Trilobites. 



It is further clear that the principle of the cessation of the 

 development of segments can only be used for articulate ani- 

 mals whose method of development is still the primitive one for 

 such animals, namely, that the segments are differentiated regu- 

 larly in front of the anal segment. This method of regular axial 

 development is often lost, owing to great specialization. In this 

 case the segments are marked off upon the surface of a germinal 

 disc. This modification would obviously allow of slight varia- 

 tions t in the inherited number of segments for each region of 

 the body, variations which would hardly be possible so long as 

 the axial method of development was retained. The Crustacea 

 are a very interesting group in this respect ; we have the pri- 

 mitive forms still developing from a JSfauplius by the regular 

 axial addition of new segments, while the higher forms, such as 

 Asfacus, mark off the segments almost simultaneously on a 

 germinal disc. 



The fact that in the Spiders the segments are marked off on the 

 germ in no way interferes with our main argument, because the 

 whole arrangement of the segmentations in the Spider-embryo 

 shows very clearly that it is only a recent modification of the 

 original method of axial development. The Spiders, indeed, like 

 the higher Crustacea, form an excellent example of animals 

 clearly derived from articulate ancestors in which, owing to 

 high specialization, the old method of regular development of 



* ' The ApodiclEe.' Macmillan. 1892. 



t Professor Howes has kindly drawn my atteution to the case of Pipa, which 

 has only eight dorsal vertebrse, whereas all the other Anura have nine. The 

 difference between the two methods of development is of great importance in 

 the question of intercalation and escalation of segments ; while it is obviously 

 impossible that these processes could take place under the primitive axial 

 method of development, they are quite possible under the later and more 

 s pecialized method 



