ON THE PELAGIC FAUNA OF PLYMOUTH FOR SEPTEMBER, 1897. 187 



As a rule three nets were used, attached to a single rope and placed 

 at different depths, the coarsest mesh near the bottom and the finest 

 close to the surface. 



I have drawn up two lists of medusae for September, one to show 

 simply the presence or absence of a species for the three different 

 years, the other to convey an idea of the abundance of medusae 

 during September, 1897. In the latter table I have given in some 

 cases the actual number of specimens taken, and in other cases have 

 conveyed a general idea of the abundance of a species by using Eoman 

 numerals as symbols. 



Most of the species given in the lists have been described and 

 figured in the Proc. Zool. Soc, 1895, and another contribution to that 

 journal is in preparation. 



The table for September, 1897, shows clearly that medusae were by 

 no means plentiful, except two species — Obelia lucifera and Phialidium 

 huskianum — and on certain days were very scarce. ]\Iost of the 

 species taken are liberated from hydroids, so that their appearance 

 and their quantity in September depend upon the breeding-time and 

 breeding-capacity of the hydroids, which are usually conspicuous by 

 their absence or scarcity. 



During my stay at Plymouth a considerable amount of dredging 

 was done in various localities, extending from the Sound to the 

 Eddystone. I examined the material carefully for hydroids, especially 

 for the minute forms, and preserved a large number of specimens for 

 further examination. The great bulk of the hydroids taken belonged 

 to genera which do not liberate medusae, such as Hcdccium, Sertularella, 

 Plumularia, etc., and the hydroids which do liberate medusae were, 

 with a few exceptions, Clyiia, Obelia, and Pcrir/onimus, scarce or absent. 

 I have noticed this scarcity of the hydroids with medusae in other 

 localities, and am not able at present to account for it. The medusae 

 of Lav sabcllarum are by no means uncommon at Plymouth and other 

 places, yet the hydroid has only been taken once at Ilfracombe, over 

 twenty years ago. The medusae of Hyhocodon frolifer have been 

 recorded from many parts of the British seas, including Plymouth. 

 They sometimes occur in vast numbers, yet the hydroid has never 

 been recorded on this side of the Atlantic, and I believe has only been 

 taken in Massachusetts Bay. The hydroid form must be somewhere 

 in the neighbourhood, as the medusa carries the ova upon the 

 manubrium until the actinula stage is reached, and this stage I have 

 taken in the tow-net. 



It is quite possible that the hydroids with medusae are really scarce. 

 A single colony is capable of liberating a vast number of medusae. A 

 colony of BougainviUia ramosa was dredged off the Eddystone in 



NEW SEKIES. — VOL. V. NO. 2. N 



