202 CHORDATA. 



BOTRYLLUS. 



The small, radially arranged colonies of this composite ascidian 

 are common on eel-grass, from which they may be separated by 

 means of a knife, and studied alive in a watch-glass with a low 

 power of the microscope. The cleaner and more transparent 

 colonies should be selected. 



1. Note the character which makes the form a "composite" 

 ascidian — the common tunic or test. Find the rnouths and the 

 common cloacal cavity. Would it be correct to say that a common 

 atrium is present? 



2. Find the annular blood-vessel and its numerous ampullce. 

 Do you observe any striking facts regarding the circulation? 

 What function have the ampullae? 



3. With your knowledge of Molgula as a guide, identify as 

 many of the organs as possible. (This is sometimes difficult 

 because of pigment.) 



4. Very young colonies, with only the first one or two genera- 

 tions of buds, may also be found on eel-grass, appearing as trans- 

 parent hemispherical lumps about a millimeter in diameter. 

 These should be fixed and stained on the eel-grass, and later 

 mounted (either still attached or removed) in balsam. These 

 will show very clearly the formation of buds of the ''parietal" 

 or ''peribranchial" type. (In this type the outer vesicle arises 

 from the integument, and the inner vesicle from the parietal 

 wall of the atrial cavity.) The inner vesicle may be seen partly 

 constricted into three divisions — the pharynx and the two atrial 

 sacs. From which ''germ layer" then are these parts in the 

 bud derived ? 



5. Look for the tailed larvae or "tadpoles" near the sur- 

 face and on the side turned toward the light, in a dish in which 

 Botryllus has been kept for an hour or two. Is this positive 

 phototrophism advantageous? Examine a larva under a micro- 

 scope. 



Draivings of the adult, the young colony, and the larva are de- 

 sirable. 



