CHARACTERISTICS OF TUNIC ATES. 215 



quently, while as a matter of fact the Tunicates, as a whole, 

 resemble, and as it were anticipate, the advent of a verte- 

 brate, like Amphioxus ; as a matter of pure hypothesis, the 

 Vertebrates may have descended from animals like Tunicates. 

 But we need not on this account dissociate the Tunicates from 

 the worms, remembering their resemblance to Balanoglos- 

 sus, and even to the Trematodes, and place them near Am- 

 phioxus and the Vertebrates in general. But we are obliged 

 to leave them at this point and pass on to the consideration 

 of the highly specialized worms and Mollusca. Crustacea, and 

 Insecta, which are circumscribed, closed types of animals 

 with possibly a vermian ancestry. 



CLASS VI. TUNICATA. 



Body usually subspherical, or sac-like, obscurely symmetrical ; some, 

 times barrel- shaped, bilateral, with a dorsal and ventral symmetry, pro- 

 tected by a transparent or dense test, containing cellulose, lined within 

 by a tunic surrounding the body-cavity. Two openings in the test, one 

 oral, the other atrial ; mouth leading into a capacious pharyngeal res- 

 piratory sac, opening posteriorly by an oesophagus into a stomach, which 

 is provided with a liver; intestine flexed, vent opening near the (esophagus, 

 the fceces passing into an atrium or cloacal space, and thence out of the 

 atrial opening. Nervous system bilateral, forming a double ganglio- 

 nated chain (Appendicularia), but usually reduced to a single ganglion, 

 situated within the tunic between the two openings ; a tubular heart, open- 

 ing at each end, lodged in a sinus-system, and its beatings often reversed, 

 the blood flowing in and out at either end. Sexes usually united ; in some 

 forms asexual individuals; reproducing by eggs or budding partheno- 

 genetically, or by gemmation. 



Order 1. Ascidiacea. Body sac-like, Bubspherical, usually sessile, 

 sometimes stalked, simple or compound, minute individuals 

 growing in a common mass ; the oral and atrial openings 

 contiguous ; often a complete inetamorpliosis. (Appendicu- 

 laria, Botryllus, Amaroacium, Clavellina, Perophora, As- 

 cidia, Boltenia, Pyrosoma). 



Order 2. Thaliacea. Body barrel-shaped ; free-swimming, test thick, 

 hyaline ; with circular muscular bands ; respiratory sac 

 widely open ; reproducing by alternation of generations. 

 (Salpa, Doliolum). 



