66 



IIIROSIII OIISIIIMA : 



with twenty-six, — both theiefore with an extra tentacle. They were 

 laid out into sections and examined. In the former, the extra 

 tentacle was found to belong to the category of the small ones and 

 was situated in thé left ventral radius. That tentacle communicated 

 with a small canal which" branched,' off dorsally from the fourth 

 tentacular canal, and not directly from the radial canal. The latter 

 specimen had the large extra tentacle in the right ventral interradius. 

 Though I was unable, on account of unsuccessful sectioning, to clearly 

 make out the origin of the tentacle in question, it seemed to belong 

 to a canal branch given off vontrally cither from" the second 

 tentacular canal or directly from "the right ventral radial canal. In 

 the two cases referred to, it is to be mentioned • that the' radial canal 

 in relation to the extra tentacle did not branch in the regular 

 normal manner, so that, f. i., the small extra tentacle of Ps. africanus 

 does not correspond to the normal fifth tentacle (textfig. 2, D 3 ) of 

 Ps. japonicus. It is not to be wondered at if there should occur in 

 the polychirotous group an abnormal form provided with supernumerary 

 tentacles, the cause of which fact might be referred to the occurrence 

 of six ambulacra as in the cases of Cucumaria piatici [Ludwig 22, 

 p. 473] and Ludivigia ocnoides (DENDY) [Reiffex, 40, PP- 615 — 716]. 

 A case in which the tentacles were evidently naturally deficient in 

 number (not as the result of mechanical loss) was presented by a 

 small individual of Pscudocucumis africanus, measuring 15 mm. in 

 length. It possessed only sixteen tentacles instead of twenty, the 

 deficiency being due to absence of a radius, or more properly speak- 

 ing, to the fact that the midventral radial canal was fused with the 

 left ventral into a common canal up to the point where the second 

 tentacular canal was given off. The existing radii, abnormally 

 quadriradial in disposition, had each four tentacles as in normal 

 cases. 



A great objection against relying upon the number of tentacles 

 in classification lies in the fact that the state of preservation of the 



