NATUKAL HISTORY OF THE GENUS DERO. 



101 



of this genus for nearly two years, have examined hundreds of 

 examples of its various species, and have done my utmost to make 

 sure of every fact which I advance. It is true that in three 

 species supplementary branchiae are present, but these are 

 much smaller than the primary ones, and are always placed at 

 the angles of the dorsal lip of the area. Where more than three 

 pairs are attributed to any species, the incurved margins of the 

 area, which are ciliated on both surfaces, have been mistaken 

 for additional branchisD. 



The integumental and mucous layers of the area are not 

 equally extensive ; the mucous layer is wanting (PI. lY. fig. 5) at 

 the tip, and encroaches on the margin of the integumental layer 

 at every other part. The branchiae are simple elevations of the 

 mucous layer, containing loops of blood-vessels one in each, and 

 are lined by a layer of stellate muscle-cells continuous with the 

 layer which underlies the mucous membrane of all the other 

 portions of the area. This muscular network is capable of great 

 expansion and contraction, and in the latter condition fully 

 realizes Bosc's description of Nais auricularis. The area is 

 extremely sensitive, the slightest jar causing contraction, and, 

 what is somewhat remarkable, it appears to be endowed with 

 sensibility to actinic light *. 



In some species, in addition to the muscular network of stel- 

 late cells already s|)oken of, an arrangement of radiating cells, 

 spindle-shaped, with forked extremities and a central nucleus, is 

 found connecting the dorsal lip of the branchial area with the 

 intestine beneath it. 



In most eases the branchial area is concave in full dilatation ; 

 but in Dero Mulleri it is capable of such expansion as to become 

 altogether convex, and to stand at right angles to the axis of the 

 body. The same is the case to a le^s extent in D. latissima. 



The chief blood-vessels are, as usual, two in number, a dorsal 

 (arterial) and a ventral (venous). The dorsal vessel in Dero can 



* I have several times tried to photograph the tail of Dero, but hitherto 

 with small success, the ignition of the magnesium-ribbon employed as 

 the source of light causing contraction in nearly every case. The same phe- 

 nomenon occurs in Hydra. In order to overcome this difficulty a drop-shutter 

 was placed between the source of the light iand the object, but the result was the 

 same ; and the marvellous rapidity of the contraction was shown by the fact 

 that, although fully expanded up to the instant of exposure, the photograph in 

 every case showed the polyp in a contracted condition. 



