10 U.S. NATIONAL MUSEUM BULLETIN 2 34 



first worker to elaborate upon this system. He expressed the rela- 

 tive size of various parts as a percentage of either the mantle length 

 or other parts. The preciseness and ease of expression of such pro- 

 portions facilitated their use in the comparison of age groups and 

 species. Robson (1929) introduced percentages in the comparison of 

 the poorly defined octopodan species and Pickford (1945) elaborated 

 upon this by precisely defining her indices and the method of measure- 

 ment. Since then Rees, Adam, and Voss in various papers have used 

 similar indices or have presented additional ones as the need arose. 



Some malacologists have criticized teuthologists for being slaves to 

 measurement and detailed description, but it must be remembered 

 that cephalopods are soft-bodied animals subject to great change in 

 shape due to different methods and degree of preservation and except 

 in a few forms no hard skeletal parts are present to preserve original 

 shape. Pfeffer (1912) has discussed many of the problems of propor- 

 tional growth and the value of proportional characters in cephalopodan 

 systematics. I do not intend to review Pfeffer's work here, but in 

 my work I have placed, with due caution, considerable reliance upon 

 proportional measm'ements in comparing like samples of closely 

 related specimens. 



Few meristic characters in squids are of value. I have not used the 

 number of suckers on the arms although this has been used by some 

 workers. Numbers of teeth on the arm and club suckers have been 

 considered, although owing to alteration with use, the shape of such 

 teeth varies considerably with age. The number of gill lamellae is 

 useful in octopods, but in squids, because of the high number and 

 wide range, it seems of dubious value. 



The radula has proved of good systematic value in the mollusks 

 in general, but I agree with Hoyle that, except in some groups, it is 

 of little reliability in the cephalopods. The differences of this struc- 

 ture as well as of the beaks do not seem to be great enough to be 

 significant. 



Primary reliance has been placed upon the numbers, position, and 

 form of the light organs, although in some species, as in Abralia 

 spdrcki, these characters may show some variability. Likewise, 

 great weight has been given to the relative size and structure of the 

 hectocotylized or nuptial arm, as well as to other sexual char- 

 acters such as size of the specially enlarged arm suckers in males 

 of Euprymna and others. The spermatophores have been investi- 

 gated where possible and give apparently sound characters for specific 

 differentiation in the males. Sucker arrangement on the tentacular 

 clubs has proved valuable in some species, and striking differences can 

 be seen in some species of Sepia and Loligo. Other characters used 

 in special cases are mentioned in the systematic section. 



