74 THE VOYAGE OF H.M.S. CHALLENGER. 



Following Verrill's example (Trans. Connect. Acad., vol. i. p. 481), I have sepa- 

 rated the genus Calliactis from Sagartia, as in it we find distinct cinclides constantly 

 present in a circle above the base. They are easily made out in a fresh state, and often 

 after treatment with reagents as warts, into which a small evagination protrudes from 

 the gastric space, so that the membrane of the wall becomes much thinned away ; an 

 opening, which it is more difficult to find, lies in the middle of the knob. If the knobs 

 do not project sufficiently above the surface, it is merely necessary to remove the uppermost 

 layer of the wall by means of a section parallel to the surface in order to make the cinclides 

 which traverse the thickness of the supporting substance distinctly visible ; this method 

 answers very well, if we wish to determine the number of cinclides in preserved specimens 

 of Calliactis. 



The forms belonging to this genus agree so far in their manner of life that they are 

 only found upon Gasteropod shells, the interior of which is occupied by a Pagurus. Their 

 best known representative is Calliactis (Sagartia) parasitica, in which my brother and I 

 have made out and described the cinclides ; other forms are Calliactis polypus, Calliactis 

 decorata, and Calliactis variegata. All these species are difficult to distinguish in a 

 preserved state, as the colour has usually formed an important point in their definition. 

 The forms of Calliactis in the Challenger material appear to me identical with Calliactis 

 polypus ; none of them belong to Calliactis parasitica. 



Calliactis polypus. 



Priapus polypus, Forskiil, Descriptiones animalium, p. 102, tab. xxvii. fig. C, 1775. 

 Cribrina 'polypus, Ehrenberg, Corallen. d. roth. Meeres, p. 40, 1834. 

 Adamsia priapus, Milne-Edwards, Hist, des Corall., torn. i. p. 280, 1857. 

 Calliactis polypus, Klunzinger, Korall. d. roth. Meeres, i. p. 76, taf'. v. fig. 1, 1877. 



Wall smooth, with a circle of 24 cinclides ; tentacles long and slender, above 600 in 

 number, placed in numerous circles, decreasing in size from within outwards ; twelve ten- 

 tacles in the innermost circle, twelve in the next, twenty-four in the third, and so on. 



Habitat.— (a) Station 208. January 17, 1875. Lat, 11° 37' N., long. 123° 32' E. 

 Depth, 18 fathoms. Three specimens on one Gasteropod shell, (b) St. Vincent, Cape 

 Verde Islands. Six specimens on one Gasteropod shell. 



Dimensions. — Breadth of pedal disk up to 4 cm. ; height up to 3 cm. 



The specimens of Calliactis polypus, taken at two different places, lay, in the one 

 case, in a group of six individuals on the shell of a Natica, and in the other in a group 

 of three on the shell of a Murex. They were, however, all contracted into a shallow 

 conical mass, at the point of which the tentacles appeared here and there, as Klunzinger 

 has abeacly described in this Actinia. 



The pedal disk is very large, and firmly fastened to the shell by means of a brownish 

 mass. The wall is smooth, and only folded longitudinally above in consequence of 

 contraction ; it is tough and opaque except in a small portion adjoining the pedal disk, 



