MiscellaneoiLs. 467 



canals which separate them. In traversing the disk the latter canals 

 also give origin to small, simple, and alternate digitiform tentacles. 

 The five radial canals of the disk cut off five sectors upon it. If we 

 examine the integument upon each of these sectors, we find it per- 

 forated with about twenty perfectly circular orifices, irregularly ar- 

 ranged, about O'OOd millim. in diameter, and bordered by an eiiithe- 

 lial ring of which the cells are 0-001 millim. in diameter. These ori- 

 fices lead into little ovoid caeca, lined with the same epithelium ; I do 

 not know what may be the function of these singular organs. The 

 very young Comaiula only presents one of them in each sector ; their 

 number consequently increases greatly with the age of the animal. 

 Some of the orifices touch each other, as if their multiplication took 

 place by a longitudinal division of preexistent caecal organs. The 

 tegumeutary membrane of the disk is hned internally with a num- 

 ber of calcareous plates, of irregularly circular form, often marked 

 with annular strife, and presenting a sort of central star thicker than 

 the plate itself, and having its arms sometimes bifurcated. Some of 

 the plates are destitute of stars ; others are perforated ; their study 

 may be of some importance in specific determinations. These plates 

 and the caecal organs just described have not previously been indi- 

 cated, so far as I am aware. 



I have made the arms of the Comatida the subject of particular 

 study. Their calcareous skeleton is formed of pieces of an hourglass- 

 shape, having at the lower part of their anterior margin a certain 

 number of spines, which prevent the complete reversal of the joints 

 upon each other. It is surrounded by a sheath of soft tissues, which 

 is developed laterally into a membranous lamella, festooned on each 

 side in such a manner that the festoons of one side alternate with 

 those of the other ; between two consecutive festoons there is always 

 a group of three unequal tentacles, the largest of which is towards 

 the extremity of the arm. These tentacles, which are all extremely 

 mobile, present no external orifice ; they bear two or three rows of 

 papillffi terminated by a small dilated head, which bears three slender 

 rigid and divergent seta^. The three tentacles of each group spring 

 by a common branch from the tentacular canal. The largest tentacle 

 exactly separates two festoons from each other ; the two smaller ones 

 repose upon the festoons, to which they partially adhere ; and this hoB 

 led Prof. Wyville Thomson to think that they formed part of it and 

 opened into the tentacular canal by a different orifice from that of 

 the large tentacle. 



The tentacular canal adheres to the vibratile epithelium of the 

 upper surface of the arms ; it is composed of two envelopes, separated 

 by brilliant stellate corpuscles ; and these two envelopes assist in the 

 formation of the walls of the tentacles. Seen in profile they simu- 

 late the appearance of two or even three superimposed vessels be- 

 neath the tentacular canal, which is the cause of the notions that 

 have hitherto prevailed as to the organization of the Comntnl(f. 

 There is, however, absolutely no other canal in the arms of the Coma- 

 tidcp, although this canal docs not rest directly upon the skeleton, but 

 is separated from it by a vacant space, which is more or less apparent 



30* 



