PLEUROBKACHIA. 45 



liant. They may frequently be seen during the daytime and can 

 often be satisfactorily observed in the shade of a wharf when the 

 water is calm. 



Unmutilated, living material can be studied to best advan- 

 tage, but preserved material may be had that is quite satisfac- 

 tory for anatomic study. 



1. In general appearance a specimen resembles a hydrozoan 

 medusa, with its ahoral surface elongated until, as a whole, it 

 approaches the shape of a fowl's egg. 



2. The broader or oral end bears two small lip-like lobes, 

 between which is the slit-like mouth. We may consider the 

 elongation of the mouth to be in the antero-posterior plane. 

 Bilateral symmetry is thus evident. 



3. At the aboral pole is the "sensory hodyJ' 



4. Leading away from this and extending as meridional 

 lines toward the oral pole are eight ctenophoral rows of swim- 

 ming plates. Examine the plates with a hand-lens and deter- 

 mine their structure and function. Determine the positions 

 of the rows with respect to the antero-posterior plane. 



5. By the sides of the stomodseum are a pair of yellowish or 

 orange tentacles that may be retracted wholly into the tentacle 

 sheath or extended through pores near the aboral pole. When 

 extended the tentacles are seen to be branched. They are very 

 sensitive and contractile. 



Digestive System. — With a pipet inject a solution of carmine 

 into the mouth opening. 



1. You can then more plainly see the long ribbon-like 

 stomodseum which extends two-thirds of the distance to the 

 sensory body, where it joins the infundihulum. 



2. From the stomodseum are given off the canals, which in a 

 successful injection will be seen to be as follows: 



(a) The axial funnel tube extending to the sensory body. 



(6) Two par agastric canals, one on each side, passing down 

 along the stomodseum. 



(c) Two tentacular canals, one on each side, passing to the 

 tentacular structures. 



