ON THE HEART OF APPENDICULAKIA FURCATA. 275 



species of the genus. There are some points, as to the mode 

 of communication between the exterior and the branchial 

 region of the pharynx, in which I do not find my notes quite 

 confirmed by M. Fol's observations, and I hope to look 

 further into that subject during the present spring. But 

 at this moment I may draw attention to a structure in 

 Appetidicularia which has not hitherto been noticed by any 

 one who has observed these interesting forms, excepting in a 

 note by myself in the * Annals of Nat. History,' 1873. The 

 matter to which I now allude is one of histological interest 

 bearing more or less directly upon the nature of transversely- 

 striped muscular fibre. It also has a wider embryological 

 interest, for I shall point out that an organ so important — 

 usually so complex — as the heart — is in Appendicularia formed 

 by only two nucleated cells, and actively functions whilst 

 consisting of no more than two ultimate units, corpuscles, or 

 plastids. The figures 6, 7, 8, in PI. XII, represent the heart 

 of A. furcata, drawn whilst under observation with a 

 Hartnack's 10, a immersion, the movement having been 

 caused to cease and the structure rendered clearer by the 

 action of a solution of Picric Acid allowed to flow under the 

 covering glass which held the specimens. 



The heart (fig. 6) is that of a smaller and less mature 

 specimen than those to which the hearts represented in 

 figs. 7 and 8 belong. It consists of two conical or 

 pyramidal cells or nucleated corpuscles, each connected 

 to the other along one edge of its broad base by fourteen 

 delicate filaments. During life these filaments are kept 

 in rapid vibration, corresponding in character to the move- 

 ment of cilia, held fast at each end, rather than to any 

 movement of muscular fibres with which we are familiar. 

 The movement is so rapid that during life the separate fibrils 

 cannot be seen, and the vibrating region connecting the two 

 conical cells has the appearance of a membranous sac. I am 

 not certain that there is not an excessively delicate mem- 

 branous connection between the fibrils, but I failed to con- 

 vince myself that there is. Even in this younger heart (fig. 

 6) some of the fibrils are seen to exhibit an alternation of 

 light and dark bands, transversely disposed. In this trans- 

 verse striation the fibrils of the adult heart exactly correspond 

 with the fibrils of the muscular mass which runs parallel 

 with the notochord in the flabelliform tail, which structure, 

 as well as other histological details of Appendicularia, is most 

 satisfactorily brought into clear definition and preserved for 

 future study by the use of Picric Acid, as above described, 

 followed by a similar introduction of the clarifying and pre- 



