30 UNITED STATES NATIONAL MUSEUM BULLETIN 29 6 



have utilized the latter course to the extent that the males are strik- 

 ingly reduced, attached to the female through most of their lives, and 

 do not compete with the female except for a very small amount of 

 burrow space. 



In discussing the conversion of the "primitive" sexual form from 

 hermaphroditism to gonochorism or vice versa, I tend to consider 

 either direction as quite possible, and it probably has occurred many 

 times in the history of the animal kingdom. Both sexual types are 

 frequently found in many species of animals and if a strong ad- 

 vantage were to be gained these species could go in either direction 

 (Tomlinson, 1966). 



I am inclined to think that the occupation of small shells and 

 small-diameter corals put such a limitation on the density of acro- 

 thoracicans, as opposed to thoracicans occupying flat rocks, that 

 the probability of encounter with another organism is small enough 

 to greatly favor a motile member of the species, namely a motile male, 

 rather than to rely on self-fertiUzation and chance contact. The 

 great densities of the cryptophialids are not to be considered here, 

 inasmuch as they are highly evolved acrothoracicans. Thus, even 

 though lack of motility favors the hermaphrodite, when the chance 

 of encounter reaches a very small level, owing not to the small numbers 

 of individuals but to their being sessile, then self-fertilization or 

 motiUty of males or gametes becomes the only means for egg and 

 sperm encounter. 



Systematics and Characteristics 



It should be noted that in the following figures all scales are 0.1 mm; 

 all specimens have been treated in KOH except the burrows; all 

 figures were made with a camera lucida except burrows and those 

 labeled "sketch." Abbreviations are on page 4. The species are 

 organized in an order reflecting natural relationships, with the type- 

 species first. Unless otherwise stated all descriptions pertain to the 

 female. 



Order ACROTHORACICA Gruvel, 1905, page 310 



[Cryptosomata Hancock, 1849, page 313; Abdominalia Darwin, 1854, page 583; 

 Gerstaecker, 1866, page 572]. 



Burrowing cirripeds with soft carapace, without calcareous plates; 

 cirri usually reduced, three to five pairs concentrated toward posterior 



