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Notes on Plymouth Sponges. 



By 

 George Bidder. 



(1) Sycon compressum : 



On a Specific Character. 



Sycon compressum is one of tlie long-recognised sponges, that stand 

 refreshingly conspicuous in a group made ditiicult with doubtful 

 definitions. The unique dermal spicules, and the striking outward 

 form, divide it from other species in a way quite different from that 

 in which Sycon raphanus is divided from S. villosum or Reniera cinerea 

 from Beniera permollis. 



Trom the doctrine of evolution it would appear the logical deduction 

 that the constant and striking differences which S. compressum shows 

 from its next allies are important to its existence, while the varying 

 differences shown among other so-called species are nearly, or quite, 

 uninfluential. It therefore seems worth while to examine whether we 

 can find circumstances in the life of the sponge which can lift this 

 statement from the deductive to the empirical standpoint, by showing 

 that there are exceptional facts in the environment, to which the 

 unusually marked specific characters exceptionally fit the animal. 



Walking on the low-tide rocks immediately mider the Laboratory 

 at Plymouth, it will be found that there occur in abundance S. com- 

 pressum, S. ciliatum, Leucosolenici hotryoides, Guancha coriacea, Hcdi- 

 clionclria pcmicea, and Hymcniacidon sangiiineum. 



In the tide-pools all four calcareous sponges occur in quantity, and 

 under heavy masses of weed both the Sycons are equally abundant. 

 But on the tops of all the naked rocks we find able to support existence 

 only the green tufts of Hcdicltondria, the red smears of Hymenicicidon, 

 and the crisp little white leaves of S. compressum. 



Both the siliceous species are comparatively massive incrusting 

 sponges, and therefore exist under completely different conditions to 

 the delicate, bag-like, Calcarea. Leaving them, therefore, for the 

 present, we find, with respect to two closely allied sponges, that 

 ^S". co7nprcss2im. and S. ciliatnm live side by side in every sheltered 

 cranny, but on the working tops of the rocks S. compressiim is alone — 



