Ch.u>. III. GENUS PLEUROBRACHIA. 241 



of those salient points, Avhich, in more or less spiral lines, beset the lassos of the 

 majority of Discophorse. 



In order to form a correct idea of the ever-changing state of the tentacular appa- 

 ratus, it is necessary to keep in mind, not only the form and structure of aU its 

 jiarts, but also their relative position in different points of view, as represented in 

 my paper in the Memoirs of the American Academy, Pis. III. and IV. The base of 

 this apparatus being attached by its flat side to the inner -wall of the cavity, appears 

 in profile, in a front view of the animal, so that the flat disk is represented by 

 a narrow margin, and its whole height is apparent. Seen from the sides of the 

 animal its width becomes distinct, and the edges, encircling its margin and rising 

 fi'om the abactinal summit of the disk along its middle line to form the pro- 

 jecting base of the tentacle, are seen in front. Seen in half profile or in a three- 

 quarter view, both margins and the tentacular base become distinct, and the ten- 

 tacle which arises from the disk can be traced from its origin along the basal 

 part of its course. In such views the whole height of the apparatus is equally 

 apparent ; but when seen from above or from below, the cavity of the tentacles 

 and the tentacular apparatus itself are shortened, and the two ch}Tniferous tubes 

 along the inner wall of the disk appear like two holes. Correctly to appreciate 

 the relations of the tentacle proper with the flat disk from which it arises, it is 

 necessary further to keep distinctly in view the arrangement of the margin en- 

 circling the disk. Along the vertical chymiferous tubes which extend to near the 

 actinal end of the tentacular cavity, there are, on each side, linear edges sHghtly 

 swollen in their middle, and cur%'ed over the middle of the disk fi'om the actinal 

 side, where they unite (PI. IP. Fig. 15) ; then extending again toward the abactinal 

 side, they are detached from the outer surface of the disk, to meet on their abacti- 

 nal margin a similar fold rising from that side, and then hang downward into the 

 bottle-shaped cavity free, as an independent thread, surrounded as soon as it is free 

 from the disk by numerous small elastic and contractile tentacles. The main 

 thread, however, fonns the stem of the tentacle, which is capable of extraordinary 

 development, and can also be contracted into a coiled buncUe ; so that, in the state 

 of utmost contraction, it forms a sort of irregular ball of tuberculated appearance 

 hanging from the hook, the tubei'cles of the surface being the lateral fringes: but 

 when elongated it is changed into a fine thread, and the fringes ajipear at inten'als 

 either in a contracted or elongated form, assuming in the former state the aj)- 

 jjearance of little tubercles, which in their elongated condition are themselves like 

 so many little threads. Their arrangement near the base of the tentacle is not 

 easily ascertained ; but when expanded, or regularly contracted within moderate 

 limits, it is evident that they all arise from one side of the main thread, and 

 are throughout unilateral 3 and where one is occasionally seen in a different position, 



VOL. III. 31 



