374 University of Califonua Publications in Zoology. [Vol.4 



other species in which aiitotomy is prevalent. There is no uni- 

 versal or general tendency for autotomy to occur at any partic- 

 ular point. Autotomy in, or near the region of major flexure of 

 the posterior horns is in some of my material more frequent than 

 that in other regions, but there is no definite zone of autotomy 

 here. This wide range in levels at which autotomy appears is in 

 itself strongly indicative that the autotomy is not due to an in- 

 ternal localized structural weakness which renders the arms prone 

 to break at given points, nor to mechanical strains of external 

 origin resulting from impact and producing localized breakage of 

 the arms as a result of the general form of the organism. 



Autotomy does not always occur in both antapicals, or in all 

 three horns, coincidently, nor does it always preserve the relative 

 lengths of the horns characteristic of the species, but the propor- 

 tion of instances in which these phases of the process are not evi- 

 dent is so small that they fall into the category of exceptions to a 

 more general rule. 



4. Significance of autotomy. — The wide-spread occurrence of 

 this phenomenon is suggestive that it is a normal physiological 

 process subject to regulation in common with growth and regen- 

 eration. It is obvious that the loss of the horns modifies the 

 specific gravity of the organism only in the event of unequal dis- 

 tribution of density of the thecal wall. Evidence upon this point 

 is only indirect. The horns are usually thinner-walled than the 

 midbody, especially in their distal portions. Empty thecae sink 

 to deeper waters as all catches from deeper levels show. The 

 specific gravity of empty thecae may be greater than that of the 

 whole organism. Autotomy leaves the thicker and presumably 

 denser parts of the theca upon the organism and in so far tends 

 to increase its specific gravity, thus tending to sink it to lower 

 levels or assisting it in retaining its position at a certain distance 

 below the surface when temperatures are lowered. 



More patent still is the change in specific surface which must 

 result from autotomy. This removes from the body the slender 

 horns, those parts in which the number of units of surface to 

 each unit of volume is greatest. By the process of autotomy the 

 volume of the organism is reduced, but proportionately less than 

 its surface. This results in a decrease in the specific surface and 



