38 Dynamic Theory 



driven forward toward the gill sac, then backward toward the hind end 

 of the intestine, so that each blood vessel becomes alternately an artery 

 and a vein. 



There is therefore no circular movement of the blood as in the verte- 

 brates. Nor is it like the circulation of the worms and insects. For in 



FIG. 57. 



FIG. 58. 



FIG. 57. Appendicularia. m. Mouth. A:. Gill intest'iie. <>. Esophagus, v Stom- 

 ach. . Anus, n. Nerve ganglia (upper throat knots), g. Ear vesicle. /.Ciliated 

 groove under tlie gill. h. Heart. I. Testes. e. Ovary, c. Notochord. ,s. Tail. (Haeckel.) 



FIG. 58. Diagram of asridian larva freely swimming. The notochord 17> is inserted 

 between the medullary tube * and the intestinal lube d and passes through the long 

 rudder-like ta 1 t<> its extremity. This tail and notochord are lost when the animal 

 reaches maturity and settles down on a rock or something and becomes stationary for 

 life, m. Medullary tube, .sm. Spinal marrow, ch. Notochord. ntp. Muscle plate, 

 a. Arms, ad. Dorsal wall of intestine, du. Ventral wall of intestine, c. Gill cavity. 

 (fteckel.) 



these the blood moves forward in the dorsal vein to the head, and back 

 through the body or the ventral vein, if there be one. In the fishes and 

 other vertebrates, the blood passes backward along the dorsal vessel. 

 So this ascidian circulation is a transitional stage between that of the 

 worm and the vertebrate, as the ascidian himself is a connecting link 

 between the worm and the vertebrate. 



The heart of the human ernbiyo is at first just such a one as the as- 

 cidian heart, but it remains a single heart a very short time, and its 

 single cavity is soon divided into two by a partition or septum. 



In this double cavity form the human emb^onic heart is the same as 

 that of a fish. And the circulation through it is also essentially fishy. 



