CTENOPHORA 



Single. Pelagic. Eight rows of meridional swimming 

 plates. No nettle cells, but with adhesive cells. With aboral 

 sense organ. This phylum consists of one class which com- 

 prises the following orders: 



Order 1. Cydippida. 



Nearly circular. Two tentacles, each of which 

 may be retracted into a sheath. (Pleurobra- 

 chia, Mnemiopsis.) 



Order 2. Lobata. 



Compressed in the vertical plane. Two large 

 oral lobes. No tentacle sheaths. (Deiopea.) 



Order 3. Cestida. 



Ribbon-shaped. Two tentacles with sheaths, 

 and numerous other tentacles. (Cestus.) 



Order 4. Beroida. 



Laterally compressed. Without tentacles. 

 (Beroe.) 



PLEUROBRACHIAi 



This form belongs to the group of animals popularly 

 called "comb jellies," and occurs along the coast in irregular 

 abundance during the summer months. Specimens are very 

 luminescent when disturbed, so, when they are abundant, 

 the display caused by them while rowing at night is some- 

 times brilliant. 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 ad- 

 vantage, but preserved material may be had that is quite 

 satisfactory for anatomic study. 



1 Although the following section is written especially for Pleuro- 

 brachia, little difficulty will be found in applying it to the related 

 genus Mnemiopsis which is usually very abundant in the vicinity of 

 Woods Hole during the late summer. 



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