CHAPTER VI 
TUNICATA 
ASCIDIACEA 
A SIMPLE ASCIDIAN (Mo/gu/a) 
Ascidians are sessile, marine animals which live attached 
to rocks, seaweed, and other objects in the waters along our 
shores. Many ascidians are colonial animals; the young indi- 
viduals, which arise by a process of budding, remaining attached 
to the parents. Ina colony which is thus formed certain organs 
are often possessed in common, and a very intimate relation is 
established between its individual members. Molgula is non- 
colonial; it is usually found in clusters attached to rocks below 
low tide. 
Molgula is a small saccular animal, an inch or less in length. 
Its outer covering is a thick, tough tunic or test, which is charac- 
terized by being partly composed of cellulose, a substance rarely 
met with in animals. The surface of the tunic is covered with 
numerous minute projections, among which sand and dirt lodge 
and cause the dirty appearance which characterizes it, except 
where it is in contact with that of other individuals. 
The animal has: two external body-openings, the incurrent 
opening or the mouth and the excurrent opening, each of which is 
at the end of a projection of the body-wall called a siphon and is 
fringed by short tentacles. The tentacles may, however, have 
been drawn into the openings and thus not be apparent. The 
incurrent siphon is at the anterior end of the body, the excur- 
rent siphon represents the morphologically posterior end; the 
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