AEACHNIDA. 639 



body and its appendages have been formed the dorsal vessel 

 appears. It is formed thus : when the division of the blasto- 

 derm into its muscular and outer layers takes place the cells 

 multiply and are heaped up along the median line of the body, 

 so as to form a sort of cordon (cord), not only in the abdomi- 

 nal, but in the thoracic region of the body. The vessel prob- 

 ably originates in the spaces between the cells, but the author 

 has been unable to trace either its origin or that of the blood- 

 corpuscles. But the rudimentary heart soon presents rhyth- 

 mic pulsations, and in the limbs we see the arteries filled 

 with a homogeneous fluid, in which can be detected the pres- 

 ence of small corpuscles, moving by impulses sjnichronous with 

 the systole of the dorsal vessel, showing that this fluid is the 

 blood. The heart already presents several dilatations (cham- 

 bers) corresponding to the abdominal segments. 



The nervous system does not appear to be formed when the 

 embryo assumes the ventral instead of the dorsal position. 

 The digestive system is verj'^ rudimentary Avhen the embrj-o 

 quits the egg. The alimentary canal is probably hollowed out 

 of the middle of the vitelline mass, being a membranous tube 

 formed around the remaining 3'olk mass. The lungs and spin- 

 nerets are well formed when the embryo is hatched, while the 

 ej'es appear later. 



The same processes of development go on in the scorpions, 

 the "post-abdomen" of the Araneina (which we have seen 

 folded back on the base of the abdomen and finall}^ to disap- 

 pear) in them being 'retained, forming the long, articulated 

 "tail;" thus the distinction into abdomen and post- abdomen 

 is very artificial as the two parts merge into each other, especi- 

 ally in Solpuga, Chelifer and Phrynus. 



In the mites the arrest of development is still more marked, 

 as the three regions of the body are in the adult not differen- 

 tiated, and the entire bod}'- assumes an oval form, the abdomi- 

 nal parts being short, thus strikingly resembling the embryo 

 of Pholcus, and the spiders generall}', as seen in Claparede's 

 figures. 



In the Acarina there is a true metamorphosis, the larvae 

 of some forms when first hatched being worm-like ; then there 

 is an oval stage when the young mite has but three pairs of 



