PLANKTON OBSERVED AT PLYMOUTH. 341 



Plymouth^ was not seen in the adult condition after this date, but 

 at the middle of September minute young Hormiphora made their 

 appeai'ance and ova were found, but the adult was absent from 

 hauls which otherwise were in excellent condition. 



The Hydroid medusa, Obelia lucifera, was very plentiful throughout 

 June. It was interesting to note the effect of killing the medusse 

 in the dark. On adding a saturated solution of corrosive sublimate 

 to the sea water the stimulus caused the animals to become phos- 

 phorescent, and the position of each medusa was indicated by a 

 small clear ring of blue light round the margin of the umbrella. 

 The light did not fade until after about a minute. 



Jime 21st. — From this day onwards Porcellana zooea have occurred 

 almost constantly. On June 28th Appendicularia were first ob- 

 served, and on July 'ith they became very common, chiefly belong- 

 ing to the species Oikopleura cophocerca. Of this form young speci- 

 mens abounded. 



Rhizoselenia obtusa and R. setigera, two diatoms with immensely 

 elongated frustules, began to occur in large numbers on July 4th. 

 On this day, moreover, Evadne appeared, together with the other marine 

 Cladoceran Podon which hitherto had been the only representative of 

 the group. Evadne gradually increased in numbers, while Podon 

 gradually became scarcer, disappearing near the middle of September. 

 Evadne constantly produced swarms of ephippial young, and is still 

 bearing ova and larvas (1st October), but about the middle of Sep- 

 tember they commenced to produce their large winter eggs, one 

 egg in each individual. Another feature of the haul on July 4th 

 was the great increase in the quantity of Dinoflagellates, Ceratium 

 tripos, C. fusus, and Peridinium sp. Both the Dinoflagellates and 

 Rhizoselenia are known to be more abundant towards the south-west 

 and near the ocean, and on June 25th I found Evadne in large num- 

 bers off the French coast, sixty to seventy miles south of Plymouth. 



The sudden appearance on July 4th of these various forms from 

 the south and south-west may perhaps be explained. On June 

 28th the wind in the western part of the channel was light and 

 variable, but from then until July 3rd — 4th it blew from a westerly 

 quarter (S.W. and W.S.W.), freshening on July 3rd. These 

 facts seem to indicate that the wind has a very marked influence 

 on the distribution of plankton. This conclusion is strongly sup- 

 ported by the fact that oceanic Radiolaria belonging to HaeckeFs 

 Acantharia also occur in the same haul of July 4th. Mr. Bourne, 

 in his Report of a Cruise in H.M.S. " Research " off the South- 

 west Coast of Ireland, remarks, " The absence of pelagic Radiolaria 

 at Plymouth has often engaged my attention,""^ and records the 

 * This Journal, vol. i, p. 321. 



