ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 41 



and forms a kind of pump in wMcli a piston is moved by powerful 

 muscles. 



In Octopus there is a sub-epithelial tissue which is connected with 

 dermal connective tissues, but in Sepia the ring is supra-epithelial, 

 and forms merely a simple thickening of the cuticular layer to which 

 it is attached by its extremities only. 



These structural differences have an effect on the mode of action 

 of the suckers, for in Octopus it is the wall of the sucker which 

 is the chief agent in the movements of fixing the apparatus, while in 

 Sepia fixation is effected by means of a central piston which works 

 independently of the rigid wall. 



In the Octopod the sucker, carried by the flexible and undulating 

 arms, is slowly conveyed towards the object which it desires to seize, 

 and the muscles which move it are highly elaborated ; in the Deca- 

 pod the arms which correspond to the eight arms of Octopus are 

 reduced ; they are, so to speak, immobile, and their suckers are 

 greatly reduced in size ; these no longer seize objects, they only hold 

 them near the opening of the digestive tube. The two tentacular 

 arms either move slightly like the rest, or are retractile and have a 

 special mobility ; even in the latter case, which is, of course, the more 

 perfect, the sucker is merely a fixing apparatus, it does not go to seek 

 the prey, it is thrust out suddenly from the cavity which contains it, 

 and the suckers are best developed at its free end. 



Retina of Cephalopoda.* — H. Grenacher finds that the retina of 

 the Cephalopoda cannot be said to be striated in the same sense as that 

 of the Vertebrata. It consists only of a single layer of elongated 

 retinal cells which, directed radially and fixed against one another in 

 palisade-fashion, represent the whole of the percipient apparatus. 

 Each cell consists of three regions, of which the nuclear is external ; 

 the region of rods consists of cells which have the form of two 

 groove-like semi-cylinders, which inclose between them the corre- 

 sponding portion of the cell ; but these halves are grouped in such a 

 way that four go to make up a rhabdome, and the two halves of the rod 

 of a retinal cell belong to different but neighbouring rhabdomes. 

 The supporting or socle-region which forms the shortest part of the 

 retinal cell is especially characterized by the rich aggregation of 

 granular pigment in its interior, and there are also cuticular invest- 

 ments more or less well developed. The nuclear region has likewise 

 a well-developed investment, and at its outer pointed end each retinal 

 cell passes into a nerve-fibre. The final portion of these fibres 

 appear, in all probability, to be represented by fine fibres which 

 extend within the retinal cell as far as the membrana limitans. This 

 last owes its origin partly to the cells which are found at the 

 periphery of the retina, and partly to the limiting cells found 

 between the retinal elements. The latter give off one to five pro- 

 cesses through the bacillar region to the side of the limiting mem- 

 brane, which is tui-ned towards the retina ; and these, notwithstanding 

 that they run within the rhabdome, are not, as was previously sup- 



* Abh. Naturf. Gesell. Halle, svi. (1884) pp. 207-56 (1 pi.). 



