98 THURLOW C. NELSON. 



medium or large. On my next visit, October 20, temperature 

 15 C., I found several great rafts of Mnemiopsis about two 

 thirds of the individuals being approximately 5 mm. long. 

 Among these there were both Phurobrachia and Bolinopsis stages 

 of development, bearing strong tentacles (Fig. 2). The smallest 

 were not more than a millimeter long. As indicating the 

 abundance of the animals, one 450 cc. fingerbowl dipped up full 

 of the surface water contained in all 36 young Mnemiopsis of 

 which 24 were in the Pleurobracliia stage and 12 in the Bolinopsis 

 stage. The majority of specimens found on subsequent visits 

 during November, December and January were in the Pleuro- 

 bracliia stage, relatively few large specimens being taken. 



In the summer of 1924 the first specimens, 6 of medium size, 

 were found July 3, water temperatures 21.5-22.7 C. Few were 

 observed after this until the 8th when, with temperatures of 24.5 

 to 24.9 C, the water was literally alive with Mnemiopsis. Most 

 of these were newly hatched embryos corresponding to Mayer's 

 Fig- 35 (Mayer, '12, PI. 6) which represents an embryo of about 

 30 hours. 



Following this heavy spawning of July 6-8 the water swarmed 

 with the ctenophores, numbers as high as 100 per cubic meter 

 being recorded. The animals continued abundant for about 10 

 days after which they diminished rapidly in numbers, and at no 

 time after this did they approximate the swaims seen in 1923. 



The lower limit of temperature at which M. leidyi will breed 

 must be very close to freezing, as the following observations show. 

 The fall of 1923 and January 1924 were very mild; our self- 

 registering water thermograph on the Maurice River, Delaware 

 Bay, rarely fell below 5 C. until late in December. The first 

 freeze in Barnegat Bay occurred on the night of January 5th, the 

 bay being covered with ice for about three days. On the morning 

 of January 5, with a temperature of 2 C., numerous medium and 

 small Mnemiopsis were procured. 4 



On my next visit to the region, January 19, no specimens were 

 taken in repeated hauls in the open bay. Medium and very 

 small individuals were fairly abundant, however, in two artificial 

 harbors about 2 meters deep which communicate with the bay by 



4 I am indebted to Mr. Wible of the Physiology Department of Rutgers Uni- 

 versity for this observation. 



