CHAPTER XIX. 



PCECILOPODA, PYCNOGONIDA, TARDIGRADA, AND LIN- 

 GUATULIDA; AND COMPARATIVE SUMMARY OF 

 ARTHROPODAN DEVELOPMENT. 



THE groups dealt with in the present Chapter undoubtedly belong 

 to the Arthropoda. They are not closely related, and in the case 

 of each group it is still uncertain with which of the main phyla 

 they should be united. It is possible that they may all be offshoots 

 from the Arachnidan phylum. 



PCECILOPODA. 



The development of Limulus has been studied by Dohrn (No. 533) 

 and Packard (No 534). The ova are laid in the sand near the spring-tide 

 marks. They are enveloped in a thick chorion formed of several layers ; 

 and (during the later stages of development at any rate) there is a mem- 

 brane within the chorion which exhibits clear indications of cell outlines 1 . 



There is a centrolecithal segmentation, which ends in the formation of 

 a blastoderm enclosing a central yolk mass. A ventral plate is then 

 formed, which is thicker in the region where the abdomen is eventually 

 developed. Six segments soon become faintly indicated in the cephalo- 

 thoracic region, the ends of which grow out into prominent appendages 

 (fig. 24.5 A) ; of these there are six pairs, which increase in size from 

 before backwards. A stomodfeum (m) is by this time established and 

 is placed well in front of t/te foremost pair of appendages 2 . 



In the course of the next few days the two first appendages of the 

 abdominal region become formed (vide fig. 245 C shewing those abdominal 

 appendages at a later stage), and have a very different shape and direction 

 to those of the cephalothorax. The appendages of the latter become 

 flexed in the middle in such a way that their ends become directed towards 

 the median line (fig. 245 B). The body of the embryo (fig. 245 B) is 



1 The nature of the inner membrane is obscure. It is believed by Packard to be 

 moulted after the formation of the limbs, and to be equivalent to the amnion of Insects, 

 while by Dohrn it is regarded as a product of the follicle cells. 



2 Dohrn finds at first only five appendages, but thinks that the sixth (the anterior 

 one) may have been present but invisible. 



