MICROSCOPICAL STUDIES IN MAEINE ZOOLOGY. 



BY JAMES HOKNELL. 



Study XVIII. — The Corynidjs. 



The family Corynidse, in its inclusion of the two distinctive 

 genera Coryne and Syncoryne, furnishes a perfect object lesson in 

 the gradations of development that prevail among Hydroid Zoo- 

 phytes ; ranging from that fullness of development characterized by 

 definite and distinct Hydroid and Medusoid stages, down to the utter 

 suppression of the latter stage and its replacement by what are mere 

 sessile bags containing the reproductive elements — degeneration of 

 the most marked description. Such gradations are always of great 

 interest and value to the evolutionist, for though the series is 

 one of degeneration rather than of progress upwards, still, it bears 

 conclusive evidence of the readiness and ease with which organisms 

 can undergo radical alteration in vital and conspicuous organs, and if 

 a species can so easily retrogress, the inference that others may as 

 readily advance by the elaboration of new organs, is logical and 

 reasonable. 



Intimate knowledge of a representative species in each of the 

 two genera referred to, is readily obtained, for both Coryne and 

 Syncoryne are present on many parts of the British Coast. 



Syncoryne, which, of the two, has the more typical life-cycle, 

 grows in littoral pools in low bushy colonies, comparatively little- 

 branched, and with a creeping stolon connecting the various main 

 stems. The latter, in S. eximia, are brown and horny, and annulated 

 only towards the base ; the twigs on the other hand are closely ringed, 

 transparent and colourless. 



The polypites are not seated in cups at the extremities of the 

 branches as in Obelia, but are naked and without any protective 

 envelope into which they can retraet upon irritation. As a natural 

 compensation, or rather adaptation, the polypites are much larger 

 and stouter, and their tentacles better equipped with stinging cells. 

 Some slight suggestion of a cup is, however, present, as the edge of the 

 chitonous tube which forms the branchlet is expanded slightly as a 

 very delicate tiny chalice at the very base of the polypite. It is 

 however of absolutely no use as a protective sheath, both on account 

 of its extreme thinness, being cuticular rather than horny or chitinous, 

 and on account of small size, only £*£ of the length of the polyp. 



