TUNICATA. 309 
respectively, at the same time the arteries are changed into veins and 
the veins into arteries, and this in consequence of the current which 
traverses these canals changing its direction after certain contractions 
of the heart. 
Simple as is their organisation, the Tunicata have a nervous system. 
It is a single ganglion, connected with divers small threads. ‘The 
organs of sensation present themselves in a very rudimentary fashion, 
Fig. 123.—Ascidia microcosmus (Cuvier). 
for after very minute search, eyes have been found. The Tunicates 
are propagated by budding, and also from eggs. The young are 
subject to some very curious metamorphoses, some particulars of 
which will be given farther on. 
Some species of the Tunicates are free, some are united to others 
in a manner more or less intimate. Hence their division into the 
three groups of simple, social, and compound Tunicates. 
The genus Ascidia is one of the best known among the simple 
Tunicates. The term is derived from the Greek word acudfov, 
‘Jeather bottle, and it has, as indicated by the name, the shape of a 
bottle or purse. The analogy becomes more evident when it is con- 
sidered that these creatures are habitually filled with water, which 
can be expelled by very slight pressure. 
Simple Tunicates attach themselves, each individual singly, to 
