High and low Dimorphism. 313 



various and uot closely related groups of aniraals, but it will be 

 noticed ihat ali my instances are drawn from the Arthropoda. I 

 do not belle ve that tbis limitation has any real significance; it is 

 simply owing to the fact that the Arthropoda in the possession of 

 a rigid exoskeleton and in the habit of chaugiug it as a whole in 

 eedysis, offer very favourable material for the Observation and 

 measurement of small diflferences in structure. I venture to suppose 

 tlierefore that the rules which we are able to formulate for this 

 class will be of general applicability. 



At the outset a definition must be given of two broadly dis- 

 tinguished types of high and low dimorphism, Avhich I bave termed 

 facultative and definitive respectively. In ali the cases which bave 

 been bitherto observed the males on the assumption of the adult 

 state cease to grow or to undergo exuviation ; consequently the high 

 and low forms which they bave assumed are permanent and do not 

 suffer any further change. Tbis I cali definitive high and low 

 dimorphism. But there occur cases in wbich growth continues after 

 the assumption of sexual maturity, and under these circumstances 

 I bave called the very remarkable kind of dimorphism wbich may 

 result, facultative, because in tbese cases the low male becomes 

 converted into the high. 



To the category of facultative dimorphism we uow tum. 



2. Facultative high and low Dimorphism. 



A. lìiachus scorpio. For more than a year a great number of 

 this crab, which is exceedingly common in the Bay of Naples, has 

 come under my Observation almost every week, so that I bave been 

 able to gain a fairly just idea of a sample of the whole population. 

 The specimens are very commonly infected with a species of /Sacew- 

 lina^ wbich was my particular object of study, but the follo wing 

 observations refer to uninfected males. In order to ascertain that 

 I was really dealing with uninfected normal males, each crab after 

 measurement was dissected and a thorough search was made for 

 the internai stages of Sacculina. These were very frequently found 

 iu a great number of crabs which sbowed no external trace of the 

 parasite, and these crabs were of course rejected from our compu- 

 tation of the normal specimens, since the preseuce of the parasite 

 profoundly affects the structure of the crab as will be shown in a 

 future work. 



21* 



