THE APODOTJS HOLOTHUBIANS 69 



not be forgotten; for this reason it is unfortunate when it is necessary to 

 describe a new species from a single specimen, but this necessity often arises. 

 As a general rule, however, we may say that any individual Synaptid belongs 

 to that species of its own genus which inhabits the region where it was taken, 

 unless it shows a marked and tangible difference, constant and without any in- 

 tergrading series, either in the form or size of the calcareous particles, in the 

 number or form of the tentacles, or in the special localization of the calcareous 

 particles in spots, rows or papillae. In case more than one species of the 

 genus occurs in the region where the individual under consideration was taken, 

 the latter will of course be compared with each one and assigned to that one 

 which it most nearly resembles, unless it shows some one or more of the above- 

 mentioned differences. In case such a difference does appear, the individual 

 is then compared with all other known species of the genus, regardless of local- 

 ity, and in case the difference is still unique, it must then be given a new name. 

 On the other hand, great care must be used to prevent confusing really dis- 

 tinct species from one side of the globe with nearly related ones from the other ; 

 on a priori grounds, it is to be assumed that a species from the Atlantic Ocean 

 is distinct from its nearest allies in the western Pacific. Even slight differ- 

 ences, if they are constant, may be used to distinguish species from widely 

 separated areas. Of course, however, it is understood that the character must 

 be sufficient to distinguish individuals of the allied species from each other, 

 regardless of whether their geographical habitat is known or not. Of char- 

 acters used to distinguish species, the most reliable and satisfactory are un- 

 doubtedly the chief calcareous particles of the body-wall, their form, size, and 

 distribution; making proper allowance for individual diversity and the possi- 

 ble solutive effects of preserving fluids, these deposits are remarkably constant 

 in any species. The number and form of the tentacles are good characters, if 

 the specimen is unquestionably mature, but differences in the form and abun- 

 dance of the lesser calcareous deposits, in the number and length of the digits, 

 in the presence or absence of a web or membrane between digits or between 

 tentacles, and in the relative length of the tentacles must all be regarded with 

 great suspicion, especially if only preserved material is available; of course, 

 where plenty of living material is at hand, some of these characters may 

 prove to be of great importance. Color and size are valuable characters in 

 some species, but very little weight can be attached to them except where a 

 large amount of living or fresh material has been examined. Several other ex- 

 ternal characters, such as roughness or thickness of the body-wall, prominence 

 of verrucae (see p. 43), and proportion of diameter to length, have been used 

 in describing species, but little or no weight attaches to them, as a rule. Of 

 internal characters, the form of the pieces of the calcareous ring is the best; 

 but even in this there is some individual diversity. The number of polian ves- 



