HEKATEROBRANOHUS SHRUBSOLII. 177 



members of the family, to a single longitudinal groove. The 

 tentacles have an annulate appearance, due to slight surface 

 ridges on which are the cilia, and to greenish-yellow streaks 

 crossing the tentacles here and there. Between the ridges are 

 short, stiff, tactile hairs. The contractile vessel (figs. 2 and 12) 

 lies freely in the cavity of the tentacle which is part of the 

 ccelom, and in which, in transparent specimens, coelomic cor- 

 puscles can be seen. The tentacles are situated more laterally 

 than in most members of the family : they are placed on either 

 side of the mouth, and slightly above it. When the animal is at 

 rest they are bent forwards in search of food, and infusorians 

 may be seen carried down by their cilia to the mouth. When 

 the animal is moving and tosses its head, the tentacles stand 

 up more or less vertically ; or, when it is moving in a definite 

 direction, they are bent back over the dorsal surface, reaching 

 back usually to the 3rd or 4th segment. 



Behind these tentacles, which, for want of a better name, 

 I have merely called " cephalic," and dorsad of them, situated 

 on the body of the 1st segment, is a pair of organs with the 

 characteristic structure of Spio branchiae, although a great deal 

 larger than these usually are (figs. 1 and 2, hr.). They are 

 about half as long again as the " cephalic " tentacles, and of a 

 reddish-orange colour, due to the presence of an ascending and 

 descending blood-vessel, forming together a simple loop in each. 

 They are ciliated, but the cilia are shorter than they are on the 

 " cephalic " tentacles, and they do not appear to be ciliated quite 

 all round. The vessels, not being contractile, are not readily 

 seen except in section (fig. 10). They run close to the epidermis, 

 projecting into the cavity of the branchia which is a prolonga- 

 tion of the ccelom. The one vessel is rather larger in calibre 

 than the other. Like the " cephalic " tentacles, they may 

 either be carried erect, or bent back over the dorsal surface. 

 Their length, also, varies much individually. Usually when 

 bent back they would cover the first five segments ; sometimes, 

 however, they reach back over more than eight. At the base 

 of each branchia are two or three short capillary chsetse (fig. 1, 

 and fig. 12, ntp^.) 



