54 LOWER VERTEBRATES. 



microscope, we find that the edges of the meshes are covered with myriads of minute 

 hairs, or cilia, which are in constant motion, forcing the water through the holes. Of 

 course the supply has to be made good, and hence more water flows in through the 

 mouth. This large cavity is known as the branchial or pharyngeal chamber, and would 

 seem to be homologous with that of the fishes. On one side of this sac (opposite to the 

 nervous ganglion), between the mouth and the bottom of the chamber, arise two folds 

 with ciliated surfaces, which pass forward and form a closed ring near the mouth, 

 which beneath the ganglion gives rise to a small process extending into the branchial 

 chamber. The other, ventral portion of this organ has glandular walls, and is known 

 as the endostyle. In the living animal, when viewed from the side, it appears like 

 a tube. Usually special gills are developed as folds of the wall of the brancliial 

 chamber. 



The water, after passing through the branchial network, is received into narrow 

 passages and conducted to a larger portion — the cloacal or atrial chamber. The 

 general relations can be seen from our diagram, illustrating a vertical and horizontal 

 section. From the atrial chamber the water flows out into the external world. 



Now we can readily see how in the older works naturalists were misled in regard 

 to the afiinities of the tunicates. They regarded the tunic as the equivalent of the 

 mantle of the molluscs (see Vol. I.), while the incurrent and excurrent openings 

 corresponded to the siphons. In one genus, Chevreulius, the resemblance was even 

 stronger, for there the tunic is in two parts, united by a hinge line, and closed by an 

 adductor muscle. How and why these views were totally erroneous will be seen when 

 we come to consider the development of these animals. 



At the bottom of' the pharyngeal sac is the narrow oesophagus, surrounded with 

 cilia which force a current down into the digestive tract. The branchial meshes serve 

 as a strainer for the water, and the larger particles which it contains fall down until 

 they are within reach of the current going down the oesophagus. After passing 

 through the throat, they come to the stomach, where digestion takes place, and then 

 the rejectamenta are carried out through the intestine and poured into the bottom of 

 the atrial cavity. 



The heart lies on the ventral side of the stomach, and is surrounded by a well- 

 developed pericardium. The most remarkable fact connected with the circulation is 

 that the heart, after beating a short time, forcing the blood through the vessels, will 

 suddenly stop for a moment and then resume its beats ; but, strange to say, after the 

 stoppage, the direction of the circulation is reversed, the blood taking an exactly 

 opposite course from that formerly pursued. This most exceptional condition was 

 first seen in the transparent Salpa, but it may be witnessed in the young of most 

 genera. We have already referred to the branchial chamber. The walls of this 

 chamber, besides acting as a strainer, are also respiratory oi'gans. The meshes of which 

 they are composed are in reality tubes through which the blood circulates, and thus 

 is brought in contact with a constantly renewed supply of fresh water. 



The nervous system in the adults of all except the CopelatEe seems to be reduced 

 to a single ganglion placed near the mouth, and thus indicating the dorsal side. In 

 forms like Cynthia it holds the same relative position with regard to the mouth, but 

 by the doubling of the body (to be explained farther on) it is also brought near the 

 atrial aperture, where it is shown in our first diagram. 



The sexes are combined in the same individual, though usually the products ripen 

 at different times. As a rule, the earlier stages of the embryo are passed inside the 



