XVIII] CIRCULATORY AND GENITAL ORGANS 403 



There is also a well-marked blood-vessel running along the inner 

 side of the genital organs on each side of the animal. These 

 vessels are connected with the subintestinal vein where it runs 

 along the liver by a series of transverse branches carried in folds 

 crossing the atrial cavity. These longitudinal veins have been 

 compared to the cardinal veins of the fishes (see p. 433) and the 

 strongest pair of transverse vessels to the ductus Cuvieri (see 

 p. 433). 



Both gill-bar and tongue-bar are strengthened with rods of 

 gelatinous tissue. These are the precursors of the visceral 

 arches, which form such an important part of the skeleton in the 

 higher Vertebrata. There are two sheaths surrounding the noto- 

 chord, an inner primary one which is produced by the cells of the 

 notochord itself, and an outer which is deposited round a hollow 

 outgrowth from the myotome called the lower sclerotome(5, Fig. 

 198). This outer sheath surrounds not only the notochord but 

 also the nerve-cord. From it are given off the myocommata, i.e. 

 fibrous septa which divide one myotome from the next. The upper 

 sclerotomes are the cavities in the dorsal fin-fold mentioned on 

 p. 400. 



The reproductive organs are very simple in construction. The 

 sexes are separate, and ovaries and tests_closely resemble each 

 other in external appearance (Fig. 197). They take the form of 

 squarish masses, which are surrounded by cavities termed gonadic 

 pouches, embedded in the outer walls of the atrial cavity. When_ 

 ripeTeggs and sperm are dehisced into the gonadic pouches. Then 

 they escape by pores which are formed at sexual maturity into the 

 atrial cavity, thence to the exterior through the atrial pore. The 

 fertilised egg develops into a free-swimming larva of a remarkable 

 form. There are no atrial folds covering the gills, but one set of 

 slits is developed long before the other, and the mouth appears on 

 the left side. It has been proved that the sexual organs are out- 

 growths from the lower ends of the myotomes, and remain 

 for some time connected with these by strings of cells (Fig. 198). 



The Hemichorda and Cephalochorda are found all over the 

 tropical and temperate regions of the world wherever a suitable 

 substratum is found. The Hemichorda burrow in mud rich in 

 decaying matter, but the Cephalochorda prefer clean sand, their 

 food as we have seen consisting of swimming organisms. 



262 



