HEART. OF THE ASCIDIAN. 433 



this tube is entirely shrivelled up, and forms a little knot of 

 nerves lying near the front above the gill-sac (Fig. 14, m). 

 It answers to the so-called upper throat-ganglion, or the 

 " brain " of other Worms. Special organs of sense are either 

 entirely wanting, or exist in the very simplest form, as 

 eye-specks and taste papillae, which surround the mouth 

 (Fig. 14, au, eyes). The muscular system is very feebly, 

 and irregularly developed. Immediately below the thin 

 leather-skin (corium) with which it is intimately connected, 

 is a thin pouch-shaped muscular membrane, as in the lower 

 Worms. On the other hand, the Sea-squirt has a cen- 

 tralized heart, and appears in this respect to be more highly 

 organized than the Amphioxus. On the ventral side of 

 the intestine, at a considerable distance behind the gill-sac, 

 lies a spindle-shaped heart (Fig. 14, kz). It permanently 

 retains that same simple pouch-shaped form which the 

 rudimentary heart of the Vertebrate possesses for a very 

 short time. (Cf. the heart of the human embryo, Fig. 

 144, c, p. 392.) This simple heart of the Ascidian, how- 

 ever, exhibits a remarkable peculiarity. It contracts in 

 alternate directions. While in all other animals the pul- 

 sation of the heart takes place constantly in a given 

 direction, usually from back to front, in the Ascidians it 

 alternates between opposite directions. First, the heart 

 contracts in the direction from back to front, then, after 

 standing still a minute, it begins to pulsate in the opposite 

 direction, driving the blood from front to back ; thus the 

 two great vessels proceeding from the opposite ends of 

 the heart act alternately as arteries and veins. This is a 

 peculiarity which appears only in the Mantle-Animate 

 (Tunicata). 



