CHARACTERISTICS OF TUNIC ATES. 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."* 



CLASS I. 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 (esophagus into a stomach, which 

 is provided with a, liver; intestine flexed, vent opening near the os&ophagus, 

 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 asexual 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 a complete metamorphosis. (Appendicu- 

 laria, Botryllus, Amarcecium, Clavellina, Perophora, As- 

 cidia, Boltenia, Pyrosoma). 



Order %. Thaliacia. 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 Salpae 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 larvae 

 are made with great difficulty, but are necessary to examine in con- 

 uection with the living, more or less transparent animals. 



* Glaus, Zoology, English edition, ii. p. 107. 



