ASCID'IAN (Gk. doRiotor, asloidion, dim. of rim*, uskos, a skinbag, a bladder), or TUNICATE (Lat. tanicalus, clothed with a. tunic). One of the small marine animals, constituting the class Uro•ho•da of the phylum Chordata, and popl in rly known as sea-scpairts. They are of t WO 1:111(1=3 : ( ) Fixed and individual ones, to which the name aseidian is more properly applied; and (2) compound free-swimming ones, 11101P prop erly designated as tunicates. Their principal in terest is the evidence they show of affinity with the vertebrates by the presence in their larval conditions of a notochord and other fundzunental characteristics of the chordata. See Plate of ASCIDIANS AND BALANOGLOSSUS.
The larva at first are free-swimming, and re semble a tadpole. The tail is fringed with a delicate transversely striated fin ; in the axis of the tail is the notochord, and above it the central nerve-cord, which anteriorly is dilated, and still farther forward again expand; into u vesicle, which is a true sense-organ—a primitive eye. The enteric canal exhibits a mouth, a phar ynx, (esophagus, stomach, and intestine; and there is a heart and blood circulation After a few hours of existence in this stage. so suggestive of an embryo vertebrate, it becomes fixed by adhesive papithe upon its 'head': its 'tail.' including
the notochord, disappears, the nervous systemic diminishes to a single ganglion, the body takes the form of a dilated sac, with an incurrent and exeurrent orifice side by side at the outer end, the reproductive organs develop, and a purely vegetative existence within such protection as is given by a leathery, contractile tunic, takes the place of the promising condition of early youth. 'there is. however, a minute animal swimming about on the surface of the sea, named Appen dicularia, which seems to belong here. and is like an aseidian larva, which preserves its chord ate condition throughout life.
These simple, solitary ascidians are hi-sexual. and increase only by spawning: but the composite tunicates form colonies by budding, as well as emit eggs, and the economy of some of these communities is very complicated. See TIALANO GLOSSITS CHORDATA ; and articles under ALI MENTARY SYSTEM, EVOLUTION OF; NERVOUS SYS 1E11. EVOLUTION OF; and similar titles, for Com parative Anatomy and relationships of the class. Consult Parker and Haswell, Text-Book of Zo ology (New York, 1S9S).