CHARACTERISTICS OF TUNICATES. 405 
in size, loses its gills and alimentary canal, while its muscu- 
lar system becomes powerfully developed. The median buds 
develop into individuals, which, resemble the sexual animals, 
except that they are without genital organs; they, therefore, 
represent a second generation of asexual forms, which become 
free and produce the sexual generation from a ventral sto- 
lon.” * 
Crass I.—TUNICATA. ‘ 
Body usually subspherical, or sac-like, obscurely symmetrical ; some. 
times barrel-shaped, bilateral, with a dorsal and ventral symmetry, pro- 
zected 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 esophagus into a stomach, which 
43 provided with a liver; intestine flexed, vent opening near the esophagus, 
the faces 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 asevual individuals ; reproducing by eggs or budding partheno- 
genetically, or by gemmation. 
Order 1. Ascidiacea. — Body sac-like, subspherical, usually sessile, 
sometimes stalked, simple or compound, minute individuals 
growing in a common mass; the oral and atrial openings 
contiguous ; often acomplete metamorphosis. (Appendicu- 
laria, Botryllus, Amarcecium, 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). 
Laboratory Work.—The Tunicates can well be studied only in a 
living state; or sections of hardened Salpse may be made. The young, 
caught with the tow-net, should be immediately examined, as they 
are very short-lived. Delicate sections of hardened eggs and larve 
are made with great difficulty, but are necessary to examine in con- 
nection with the living, more or less transparent animals. 
* Claus, Zoology, English edition, ii. p. 107. 
