NOTES ON THE FATJNA OF FALMOUTH. 205 



The first Chrysaora isosceles was seen drifting up the harbour 

 on the flood-tide, , on the 11th July, 1898, and during the next 

 month of both years this species was fairly common. The largest 

 specimen of species was detected in the harbour on the 23rd 

 August, 1899. It was found to measure 27 "50 cm. in diameter. 



The ambulatory gonozooids of Cladonemma radiatum and 

 Clavatella prolifera abounded in the tidal pools exposed during 

 low water in all the sheltered creeks of Falmouth harbour and 

 Helford river. As in former years, the hydroid of both species 

 has so far eluded my most careful search. 



Teematodes. The parasitic trematode, whose presence I 

 have detected in the structureless layer of the umbrella of several 

 species of medusae during the past few years, was found during 

 the summer of 1898 to be very numerous within the digestive 

 sacs of the common hydroid, Cljdiia Johnstonii ;occuring on drift 

 Zostera. WhUe examining a tow-net gathering made on the 

 11th August of that year, I happened to find a small piece 

 of Zostera marina about 10 m.m. in length; and as most of 

 the hydroids on this weed seemed to be abnormally swollen, 

 I examined it under the microscope. Great was my astonish- 

 ment to find that almost every polypide had, in the majority 

 of instances, two, and in some cases, three of these trematodes 

 wandering about within the digestive sac. A few days 

 later, I collected during low water from various parts of the 

 harbour some pieces of Zostera from the beds on which they were 

 growing ; and although these ribbon-like leaves were covered 

 with specimens of this hydroid, in no instance could I find a 

 single trematode within any of them, although on the drift speci- 

 mens they abounded. During this time, medusae were scarce, 

 and it seems to me most probable that these parasites were 

 compelled to find a new host wherein to undergo further 

 developmental changes, and this may account for the singular 

 circumstances under which they were found. I may add that 

 these hydroids seemed to be in no way inconvenienced by the 

 presence of these trematodes, for they expanded fi'eely when 

 under observation. 



MoLLUSCA. All species of Nudibranchs were scarce during 

 both years. 



