Comparative Development. 37 



When the human embryo develops the notochord, the forerunner of 



the backbone, it imitates the appendicularia, and the embryo or larval 



form of the seasquirt ascidian, which are the lowest animals pos- 



FIG. 56. Balanoglossus, a young acorn worm. (Alex. 

 Agassiz.) 7. Proboscis, h Collar. &. Gill arches and gill 

 openings of anterior intestine, a long row on each side. d. 



Digestive posterior intestine filling the greater part of the 

 body cavity, v. Intestinal vessel lying between two paral- 

 lel folds of the skin, a. Anus. 



sessing the notochord, although the amphioxus is 

 reckoned the lowest vertebrate now in existence. 

 The supposition is that some worm-like animal, 

 unlike any of these three, and now extinct, was 

 the original possessor of the notochord, transmit- 

 ting it to a posterity from which diverging forms 

 have arisen. 



The essential parts of the appendicularia are 

 the mouth, the gill intestine, the oesophagus, the 

 stomach and the anus, altogether forming the 

 opening through the body; a simple tube shaped 

 heart, some simple knots of nerve ganglia, a 

 simple ear vesicle, long tail with notochord or 

 rudimentary backbone, and testes and ovary (her- 

 maphrodite). The ascidian, another tunicate ani- 

 mal, has a development similar to that of the 

 appendicularia. In the larval state the ascidian 

 is a freely swimming animal, with 'a tail contain- 

 ing a notochord like the appendicularia, and a 

 rudimentary medullary tube, but when it reaches 

 maturity it loses its tail and notochord, the 

 medullary tube shrivels up forward into a per- 

 manent bunch of nerves called the throat gan- 

 glion, and the animal becomes fixed to a rock, or other stationary object, 

 by its tunic. This is a wood-like, cellulose case of leathery consistence, 

 which grows around the animal. In this case the soft animal, instead 

 of lying or standing at full length, is doubled upon itself so that the 

 end of the intestine with its anal opening, points upward. Its dis- 

 charge is into the cavity of the tunic, and the outlet from the tunic 

 is at its top, near the animal's mouth. So it stands, as it were, up- 

 on the middle of its intestine. Near this middle is the heart, which 

 is an organ with a single cavity. From the heart a single blood ves- 

 sel extends forward or upward to the gill sac, and another, from the 

 other end, extends upward in the direction of the intestine, which is the 

 same as backward. The heart has a slow pulsation, contracting first at 

 one end and then the other, by which movement the colorless blood is 



