PHYLUM CHORDATA 

 SUBPHYLUM TUNICATA 



Class: 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 individuals, 

 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 estab- 

 lished between its individual members. Molgula is noncolonial ; 

 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 nu- 

 merous 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 in- 

 current siphon is at the anterior end of the body, the excurrent 

 siphon represents the morphologically posterior end ; the portion 

 of the body lying immediately between the two is the dorsal side ; 

 the opposite side, which is very much longer and includes the sur- 

 face of attachment, is the ventral side. A stream of water is drawn 

 into the incurrent opening, bearing the minute organisms which 

 constitute the animal's food and the air needed for respiration ; 



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