362 



IIsrVERTEBEATA 



CHAP. 



splits in the mesoderm. The coelomic cavities contain a clear fluid, 

 and have a definite epithelial arrangement of the cells forming their 

 walls ; whilst the blood spaces contain an albuminous serum which 

 stains, and their walls are often irregular and in some places formed 

 only by the yolk-membrane. 



The coelom arises as two vesicles lying beneath the mid-gut. 



Each of these vesicles 

 becomes divided by a con- 

 striction into a dorsal part, 

 which is the rudiment of 

 the pericardium, and a 

 ventral part, the rudiment 

 of the kidney ; the connec- 

 tion between the two, 

 though narrow, persists, 

 and forms the reno-peri- 

 cardial canal. 



Beneath the shell-sac, 

 on the posterior slope of 

 the embryo, a wide blood 

 space arises whose cavity is 

 traversed by cords of mesen- 

 chyme. This is the posterior 

 sinus (Fig. 294). It ex- 

 tends anteriorly round the 

 sides of the gut and there 

 constitutes the two forks 

 of the vena cava. From 

 this a branch extends into 

 the rudiment of the gill, 

 which is the beginning of 

 the branchial heart. In 

 front, these forks unite 

 beneath the gut to form 

 the unpaired vena cava. 

 The heart arises independ- 

 ently of this sinus as two 

 hollow tubes lying inter- 

 nally between the coelomic 

 rudiments. Behind, these 

 tubes unite to form the ventricle, but in front, where kidney and 

 coelom join, they diverge so as to form the auricles (Fig. 295). 



In the next period of development, the end of which is represented 

 in Fig. 296, the embryo becomes as large as the external yolk- 

 sac, and the funnel is definitely constituted by the union of the free 

 edges of the folds in the mid-ventral line. The arms have now 

 acquired suckers and have extended round the head to the mid- 

 dorsal line, so that the encircling of the head by the fore foot is 



Fig. 294. — Two diagrammatic transverse sections 

 through a young emliryo of Loligo vulgaris to 

 illustrate the origin of the coelom and the blood 

 cavities. (After Faussek.) 



A, The more posterior ; B, the more anterior section. 

 6r, gill ; ?)r.7i, rudiment of branchial heart ; coe, rudiments 

 of coelom ; H, rudiments of systemic heart ; int^ intestine ; 

 ji.s, posterior blood sinus ; s.s^ shell-sac ; v.c, vena cava. 



