CHAPTER VI 



UROCHORDA (TUNICATA) 

 ASCIDIACEA 



A SIMPLE ASCIDIAN. MOLGULA 



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. In a 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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