64 LOWER VEUTEBRATES. 



eye. Pairs of nerves are rooted in the upper jjart of the sides of the cord, from wliicli 

 they pass out and down to be distributed to the various organs. In nunilier the pairs 

 are the same as the plates or bands of muscles. 



Colored blood is not found in the lancelet ; the corpuscles are white and few in 

 number. The heart is a bulbous expansion of tiie principal vein (vena cava), and lies 

 beneath the first branchial arches. The vein is contractile ; it extends from the vent 

 to the heart below the alimentary canal, below and above the liver. From each side 

 of the portion beneath the pharynx, vessels carry the blood to the gills ; above the 

 latter, on each side, it is received in an arteiy which carries it back to the oesophagus, 

 where these arteries unite in a single distributing vessel, which continues to the end 

 of the body. From the main heart two small vessels supply the blood for the mouth, 

 and a third that for the pharynx. How tiie blood is returned from the arterial system 

 to the veins has not yet been satisfactorily determined. 



Along each side of the branchial chamber, at the lower edge of the lateral series of 

 muscles, the generative organs are jilaced. They form two series of about twenty-five 

 little sacs or cases each. Within each of these is developed a quantity of sperm in the 

 male or eggs in the female. When the eggs are near maturity, the sacs are very notice- 

 able in females. On ripening, the contents are set free in the liranchium bj- the breaking 

 up of the walls of the cases, and from this chamber they escape by the branchial jiore, 

 or, according to some authors, through the gill openings into the pharynx and out of 

 the mouth. The eggs are fertilized by sperm set free in the water. It is possible that 

 fertilization may take place in the branchium. 



The develojiment of the egg is verj- interesting from a scientific standjwint, and 

 has been followed by Kowalevsky, Hatschek, and, in our own counti-y, b_y 11. J. Rice. 

 Segmentation is not interfered with by food-yolk in the egg. It results in a complete 

 blastosphere (blastula). This blastula has vibratile cilia, and rotates within the envel- 

 oping membrane. One side gradually thickens, and, folding inward, becomes the 

 hypoblast, and the embryo, taking on the gastrula form, soon escapes into the water. 

 Here, swimmin" about by means of the vibratile cilia, it becomes more elongate and 

 cylindrical. The edges of the cup join from in front backwards, leaving a single 

 opening behind. E])ib]ast and hyjjoblast approximate along the dorsal axis, and the 

 mesoblast forms as outgrowths of the primitive stomach between them. A medullary 

 plate appears above the notochord, a groove foi-ms upon this, and, later, is converted 

 into a neural canal. After the cavity of the abdomen is formed by the closing in of 

 the sides of the gastrula, the mouth is formed as a narrow slit ; as develojiment jiro- 

 ceeds, it widens, occupies a more anterior position, and becomes surrounded by the 

 tooth-like tentacles. The gill openings appear behind the mouth one after another. 

 With the apjiearance of these clefts, folds from the ectoderm grow downward on 

 each flank, and, uniting beneath, form the ventral fin and enclose the branchial 

 chamber in front of it, leaving the ]wre open. Others of the more prominent changes 

 in the early stages are the forward growth of the li\er from beneath the stomach, 

 and the development of the cartilaginous supports of the fins. The very young are 

 quite transparent. 



Specimens ,it hand from the Gulf of Mexico differ considerably from others col- 

 lected in the Mediterranean. The European species has been named Brancldostoma 

 lanceolatnm,the American j5. caribrpirm. In all six species are known from the whole 

 ■world according to Dr. Gtinther, of the British Museum. One form, from Australia. 

 has been regarded as the type of a distinct genus, Epigionichthys. 



S. Gaeman. 



