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 sj'nchronous with 

 the s}*stole 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 very rudimentary when the embryo 

 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 yolk mass. The lungs and spin- 

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

 eyes 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 finally 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 mei'ge 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 body assumes an oval form, the abdomi- 

 nal parts being short, thus strikingly resembling the embryo 

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

 figures. 



In the Acarina there is a true metamorphosis, the larvas 

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

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





