ritter: octacnemus. 241 



the entire auimal, to the angles of which the radial muscles are attached. 

 He remarks that "opposite the indentations in the margin of the thick- 

 ebed central portion of the membrane, the thin lamina [which is de- 

 scribed elsewhere as a continuation of the horizontal membrane] is loose, 

 and hangs in bags or depressions." The condition here described is, I 

 gather, what gives one the impression from his figures that the oral disc 

 is occupied by a great sac that extends to the very base of the arms, 

 even bellying into the arms somewhat, and is quite distinct from the 

 octagonal central area to whicli the eight radial muscles are attached. 

 In other words Moseley found the sac in his animal considerably more 

 complicated than it is in that now uuder examination ; and the points 

 brought out by him are of such a character* as to justify the belief that 

 he was dealing with a different creature. This is another of the par- 

 ticulars "which persuades me that two species of Octacnemus should be 

 recognized. 



Herdman refers to the mantle in the specimen studied by him as 

 adhering closely to the inner surface of the test. As, however, this 

 statement is made in close coimection with what he says about the 

 musculature, I judge he refers only to the mantle within the arms. 

 Herdman describes a number of pits in the horizontal membrane, the 

 significance of which he was in doubt about, but which he conjectures 

 may represent branchial stigmata. I have seen a few of what may be 

 the same structures, though I fail to make out that the}^ are as definite 

 or as numei'ous as they were in the specimen studied by Herdman. I 

 have no suggestion as to their meaning, but they certainly cannot be 

 homologous with branchial stigmata. 



The cavity below the horizontal membrane, which is immediately con- 

 tinuous with the great chamber occupying almost the whole of the ani- 

 mal, I regard, with Moseley and Herdman, as probably atrial, or atrial 

 and peribranchial. This cavity is without partitions, so far as I have 

 observed. It, of course, opens to the outside world through the atrial 

 aperture, which is nearly circular and without distinguishable lobes or 

 markings of any kind. The view that this chamber is atrial is borne 

 out by the fact that the " nucleus" (in reality, as we shall presently see, 

 the whole viscera proper) is so loosely suspended within it. Two or 

 three facts however, to be pointed out presently, throw some doubt on 

 the cori'ectness of this interpretation. 



We may now turn to the examination of the visceral mass. The ex- 

 act position within the test envelope which this occupies in life is by no 

 means clear. In all my specimens, as with those taken by the " Chal- 

 voL. XLvi. — No. 13 16 



