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 

 compartments. 

 The somatic 



FIG. 206. TRANSVERSE SECTION THROUGH THE THO- 

 RACIC REGION OF AN EMBRYO OF ACELENA LABYRINTHICA. 



The section is taken from an embryo of the same age 

 as fig. 201 A, and passes through the maximum pro- 

 tuberance of the ventral yolk sack. 



i'n. ventral nerve cord ; yk. yolk ; nie.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. 



