452 



ARACHNIDA. 



are developed widish spaces between the layers of somatic 

 and splanchnic mesoblast, which form transversely directed 

 channels passing 

 from the heart out- 

 wards. They are 

 probably venous. 

 At a later stage 

 the septa send out 

 lateral offshoots, 

 and divide the 

 peripheral part of 

 the abdominal cav- 

 ity into a number 

 of compartments 

 filled with yolk. It 

 is probable that 

 the hepatic diverti- 

 cula are eventually 

 formed in these 



me.s 



FIG. 206. TRANSVERSE SECTION THROUGH THE THO- 

 RACIC REGION OF AN EMBRYO OF AGELENA LABYRINTHICA. 



compartments. 

 The somatic 



The section is taken from an embryo of the same age 

 as fig. 20 1 A, and passes through the maximum pro- 

 tuberance of the ventral yolk sack. 



mi. ventral nerve cord; yk. yolk; me.s. mesoblastic 



layer of mesoblast somite ; ao - aorta - 



is converted into the muscles, both of the limbs and trunk, the 

 superficial connective tissue, nervous sheath, etc. It probably 

 also gives rise to the three muscles attached to the suctorial 

 apparatus of the oesophagus. 



The heart and aorta are formed as a solid rod of cells of the 

 dorsal mesoblast, before it is distinctly divided into splanchnic 

 and somatic layers. Eventually the central cells of the heart 

 become blood corpuscles, while its walls are constituted of an 

 outer muscular and inner epithelioid layer. It becomes func- 

 tional, and acquires its valves, arterial branches, etc., by the 

 stage represented in fig. 201 B. 



The history of the mesoblast, more especially of the mesoblastic somites, 

 of the Scorpion is very similar to that in Spiders : their cavity is continued 

 in the same way into the limbs. The general character of the somites 

 in the tail is shewn in fig. 207. The caudal aorta is stated by Metschnikoff 

 to be formed from part of the mesenteron, but this is too improbable to be 

 accepted without further confirmation. 



