280 Miscellaneous. 



I ascertained at once that I had to do with a form of the genus 

 Acineta. "We find this Acineta soHtary and attached by means of a 

 slender peduncle to the various Algae ; and it is tolerably abundant. 

 The test, protoplasm, and nucleus did not greatly engage my atten- 

 tion ; but the suckers are very remarkable : in fact, instead of being 

 of a certain number, collected into bundles and arranged symmetri- 

 cally on one side and the other of the body, or distributed over the 

 whole of the free surface of the protoplasm corresponding to the 

 aperture of the test, they are only two in number, placed opposite 

 one another. Whilst in the other Acineta3 these sucking-tentacles 

 are slender, more or less long, and usually rigid, in this they are 

 flexible in aU directions and very mobile, so that we see them move 

 and twist about continually. 



In accordance with these remarkable characters, which I have not 

 met with in any Acineta hitherto described, I think I may establish, 

 if not a new genus (so as not to complicate further the divisions of 

 this group), at least certainly a new species, under the name of 



Acineta dibdalteria, sp. nov. 

 Diagnosis. Test in the form of a wine-glass ; peduncle slender ; 

 tentacles of a single kind ; protoplasm granular, more transparent 

 at the periphery ; contractile vesicle large ; nucleus in the shape of 

 a horse-shoe, and placed towards the lower part of the protoplasmic 

 mass. Only two tentacles, which are at the same time suctorial 

 and prehensile, movable in all directions ; peduncle straight, 

 slender, of uniform diameter, and only a little widened towards the 

 base to attach itself more firmly to the plant which bears it. 



Dimensions. 



millim. 



Transverse diameter of the test (maximum) 0-06 



Vertical diameter of the test 005 



Length of the peduncle 003 



Breadth of the peduncle 001 



Length of the suckers 0-04 



From these characters it seems to me that this form cannot be 

 confounded with its congeners. In fact, if we run over the figures 

 of those which are at present known, we shall see that none of 

 them approaches the species just described. In reality there is no 

 Acineta that presents tentacles reduced to two only. 



We have here a very remarkable example of anatomical and func- 

 tional retrogression. The organs having been reduced, the functions 

 have been concentrated. The differentiation of the suctorial and pre- 

 hensile tentacles having ceased or being absent, the two correlative 

 functions have been compelled to combine in the same organ, which, 

 in its turn, in order the better to perform its now multiple part, 

 has been obliged to modify and adapt itself. We have evidence 

 that this must have taken place when we find that whilst in the 

 other Acinetse the tentacles are usually rigid and motionless, in 

 the present case, as we have already said, they are flexible and 

 movable in all directions. — Bibliotheque Universelle ; Archives des 

 Sci. Phys. et Nat., February 15, 1881, p. 181. 



