MNEMIOPSIS. 37 



CTENOPHORA. 



MNEMIOPSIS. 



This form belongs to the group of 'animals popularly called 

 "comb-jellies," and occurs along the coast in irregular abun- 

 dance during the summer months. Specimens are very phos- 

 phorescent when disturbed, so, when they are abundant, the 

 display caused by them while rowing at night is sometimes bril- 

 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 aboral surface elongated until, as a whole, it 

 approaches the shape of a fowl's egg. 



2. The broader or oral end bears two heavy terminal lobes, 

 between the bases of which is the slit-like mouth. We may con- 

 sider the elongation of the mouth to be in the antero-posterior 

 line. A bilateral symmetry is thus evident. 



3. On each lateral surface of the animal, midway between 

 the terminal lobes, at a short distance from the mouth, notice a 

 small opaque spot, an undeveloped tentacle in its sheath. 



4. At the aboral pole is a depression, in the bottom of which 

 is the "sensory body." 



5. Leading away from this and extending as meridional 

 lines toward the oral pole, are eight ctenophoral rows, or swim- 

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

 mine their structure and function. A pair of rows (arising 

 from the pit of the sensory body as a single row) extends down 

 over each terminal lobe. Another pair passes down each lateral 

 surface of the animal. Each of these lateral rows, after passing 

 half-way to the mouth, changes its appearance somewhat, and 

 leaves the surface of the body proper, being continued down one 

 edge and up the other of a finger-like process which hangs orally, 



