52 VERTEBRATE ZOOLOGY 



The heart is no less than an oddity. It is a pulsating tube, lying 

 ventral to the stomach, open at both ends into sinuses. It works by 

 peristaltic contractions first in one direction for a few beats and then 

 in the other. The colorless blood is thus driven through sinuses first 

 toward the anterior and then toward the posterior organs. No other 

 such heart is known among animals. 



All ascidians are without exception hermaphroditic, as befits their 

 sessile life, though there is a high degree of self-sterility. Usually 

 the eggs of one individual meet the sperm of another in the sea-water. 



An adult ascidian does not therefore appear to be an impressive 

 candidate for the honor of being classed as a chordate, but it makes a 

 much better showing when we consider the entire ontogeny. 



Development and Metamorphosis of an Ascidian 



The ascidian egg is small and almost yolkless. The cleavage 

 stages, blastula, and early gastrula are essentially similar to those of 

 Amphioxus, except that the embryonic life is longer. The gastrula 

 is not a ciliated larva but a passive embryo; in fact the embryonic 

 development is greatly prolonged and the larval life does not com- 

 mence until the egg hatches and the advanced tadpole larva emerges. 



ine^ air sens.vetl eiLaf 



Fig. 24. — Anterior end of the "tadpole" larva of an ascidian {Ascidia mam 

 millata) in optical section, adh, adhesive papillae; ali, rudimentary alimentary 

 tract; atr, atrial aperture; cil. gr, ciliated diverticulum, becoming ciliated funnel; 

 end, endostyle; eye, eye; med, nerve-cord; nolo, notochord; oto, otocyst; sens, ves, 

 sense- vesicle (cavity of brain) ; stig, earliest stigmata or pharyngeal clefts. (From 

 Parker and HaswfeU, after Kowalevsky.) 



The Ascidian Tadpole. — When we examine the larva of the tunicate 

 (Fig. 24) we understand why the Urochordata have been classed with 

 the chordates. The notochord is quite typical but extends only as 

 far as the pharynx, or, as the name Urochordata impUes, is confined to 

 the tail. The tail is also characterized by having dorsal and ventral 

 fins and a neural tube. The brain or sense vesicle is much larger than 



