viii ARTHEOPODA 229 



These gaps are the origins of the ostia of the heart. The coelomic 

 sacs belonging to the cephalic lobe also grow in a dorsal direction. 

 They then become divided into dorsal and ventral portions. The 

 latter lose their cavities and disappear, being probably transformed 

 into connective tissue. The former meet in the mid-dorsal line and 

 enclose between them a space which forms the dorsal aorta. 



The coelomic cavities belonging to the six segments of the 

 prosoma do not grow dorsaUy but extend further and further into the 

 appendages, as these grow longer and become divided into joints. 

 Soon the greater part of the cavities which are contained in the 

 appendages disappear, their walls becoming converted into the 

 extensor and flexor muscles which move each segment of the limb. 

 The coelom disappears entirely in the segments belonging to the 

 chelicerae and pedipalpi, but in the four segments which bear the 

 ambulatory legs a small remnant persists in the base of each leg as a 

 tiny vesicle. That portion of the coelom which is situated in the 



Fig. 178. — Longitudinal section through part of the abdomen of Agelena labyrinihica, 

 in order to show the origin of the genital organs. (After Kautsoh.) 



3, 4j and 5, the coelomic sacs ot abdominal segments B, 4, and 5 ; gon, rudiment of genital organ ; 

 v.n, thicltened ectoderm which gives rise to the ventral nerve cord. 



base of the first leg becomes connected with an ectodermic invagina- 

 tion which grows in to meet it. This invagination forms the duct of 

 the coxal gland ; the glandular portion of the organ is formed by the 

 union on each side of all the coelomic sacs belonging to the four 

 walking legs. This excretory organ may therefore be compared to 

 one of the coelomiducts or so-called " nephridia " of Peripatus. The 

 coelomic sacs which were situated in the first abdominal segment 

 disappear; those which existed in the sixth, seventh, and eighth 

 segments fuse into a single cavity on each side, which becomes 

 pressed into the caudal lobe by the same force which presses the yolk 

 mass downwards. 



The genital organs make their appearance as large pale cells in 

 the walls of the coelomic sacs of the third, fourth, and fifth abdominal 

 segments (Fig. 178). When the cavities of these sacs disappear 

 these cells coalesce to form a compact mass on either side of the body 

 which is the rudiment of the genital organ. The genital duct is 

 formed by an outgrowth from the sac of the second abdominal segment 

 and therefore, like the duct of the coxal gland, it is a coelomiduct. 



