64 LOWER VERTEBRATES. 



eye. Pairs of nerves are rooted in the upper part of the sides of tlie cord, from which 

 they pass out and down to be distributed to the various organs. In number 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 the 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 livei-. 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 artei-y which carries it back to the cesophagus, 

 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 month, 

 and a third that for the pliarynx. How the blood is retui-ned 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 placed. 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 tlie eggs are near maturity, the sacs are veiy notice- 

 able in females. On ripening, the contents are set free in the brauchium by the breaking 

 ujj of the walls of tlie cases, and from this clianiber they escape by the branchial pore, 

 or, according to some authors, througli the gill ojieniugs into the phar^ynx and out of 

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

 fertilization may take place in the branchium. 



The development of the egg is very interesting from a scientific standpoint, and 

 has been followed by Kowalevsky, Hatschek, and, in our own country, by H. 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, becoines tue 

 hypoblast, and the embryo, taking on the gnsti-ula form, soon escapes into the watci-. 

 Here, swimming 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. Epiblast and hypoblast 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 forms upon this, and. Inter, is converted 

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

 the sides of the gnstnila, the mouth is formed as a narrow slit ; as development pro- 

 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 appearance 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 pore ojaen. Otliers 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 at hand from the Gulf of Mexico differ considerably from others col- 

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

 lanceolaUim, the American J3. caribcp.iim. In all six species are known from the whole 

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

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



S. Garman. 



