470 The Tunicates, or Ascidians 



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 pharnygeal 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 

 ejectamenta 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 re- 

 markable 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 circu- 

 lation 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 organs. 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 central nervous system in the adults of all except the 

 Larvacea is reduced to a single ganglion placed near the mouth 

 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 further on) it is 

 also brought near the atrial aperture, where it is shown in 

 our first diagram. 



Development of Tunicates. 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 cloacal chamber, though in some the development 

 occurs outside the body. As a type of the development we 



