M CTENOPHORA. 



how the polyps are placed. The structure of the expanded 

 polyps is nicely shown by Renilla. The spicules of such forms 

 as Gorgonia may be obtained by boiling a portion of a colony 

 in caustic potash. What purpose do such spicules serve ? 



Parker: The Reactions of Metridium to Food and Other Substances 



Bui. Mus. Comp. Zool. Harvard, 29, 1896. 

 : The Mesenteries and Siphonogljq^hes in Metridium marginatum. 



Bui. Mus. Comp. Zool., Harvard, 30, 1897. 



Longitudinal Fission in Metridium marginatum. Bui. Mus. Comp. 



Zool., Harvard, 35, 1899. 



The Reversal of the Effective Stroke of the Labial Cilia of Sea- 



Ajiemones by Organic Substances. Am. Jour. Physiol., 14, 1905. 



^:-: The Origin and Significance of the Primitive Nervous System. 



Bui. Mus. Comp. Zool., 50, 1911 

 — : The Elementary Nervous System. Lippincott, 1918 



CTENOPHORA. 



Single. Pelagic. Eight rows of meridional swimming plates. 

 No nettle cells, but adhesive cells. With aboral sense organ. This 

 phylum consists of one class which comprises 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.) 



PLEUROBRACHIA. 



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- 



