Hydroida from Deep Water. 127 



closely allied to the well-known A. iiiijrwjJiijUuin of our own 

 coasts, upon which, indeed, it is a very slight variation. In 

 the shape of the calycles and in the minute structure generally 

 the two are almost identical. The differences lie chiefly in 

 size and habit, the points most readily affected by change of ex- 

 ternal conditions. The northern form is dwarfish as compared 

 with our own j it looks stunted, and wants the elegant plumose 

 appearance so characteristic of the latter. The stem, too, is 

 simple and slender, instead of being composed, as in the normal 

 A. myi'iojyhyllum, of several tubes bound together. One other 

 difference between the two exists : in A. radicellata a large 

 number of long and much-branched fibres are given off from 

 the base of the stem, which form a spreading root-like appen- 

 dage ; by this the zoophyte fixes itself in the muddy ground, 

 from which it Avas obtained. This, however, it must be 

 remembered, is only an exaggerated condition of a structure 

 which exists in A. myriophyllum. The latter zoophyte rises 

 from a tuft of tangled fibres, a mere means of attachment 

 when it grows on solid substances, but which, we can readily 

 suppose, might be developed into a root-like organ under 

 other conditions of life. The corhula oi A. radicellata seems 

 to resemble that of ^. myriophyllum. The similarity of the 

 two forms in minute sti-ucture * is really very striking, and the 

 differences between them are just those which a change of 

 locality might most readily produce. 



The second species described by Sars {A. Mcuspis) exhibits 

 more distinctive characteristics ; but its calycles are clearly of 

 the myriophyllum type. It is somewhat larger than the pre- 

 ceding, but does not exceed a couple of inches in height. Tiie 

 stem is compound towards the base ; and the pinnulated portion 

 is much larger than in A . radicellata. The calycles in shape 

 and arrangement resemble those of the last-named species 

 andof ^. myriophyllum'^ but the minute crenulation of the 

 rim is wanting, and the central tooth on the front margin, 

 which in A. radicellata was truncate and slightly sinuated at 

 the top, is here divided down the middle so as to form two 

 acuminate processes^ which have suggested the name of the 

 species. But the most remarkable peculiarity of ^. A/f«.s;?<s 

 is foimd in connexion with the gonosome. The reproductive 



• Sars aeems to consider that the margin of the calycles is more 

 diitinctly crenulated in the Norwegian than in the British form ; but the 

 difference, if there is any, is of the slightest. Both are crenulated ; and 

 the crenations are of precisely the same character. A difference, however, 

 does exist in the character oi the anterior tooth, which in A. mijriophyllinn 

 is pointed, in the northern form truncate and slightly sinuated at the 

 apex. 



10* 



