2 PROCEEDINGS OF THE NATIONAL MUSEUM vol.73 



Drepanorchis. In these species the mesentery is complete, as in Sac- 

 culina; the testes, however, have a curved shape, as in Drepanorchis. 

 They differ from Sacoulina as well as from Drepanorchis in another 

 anatomical respect: The visceral mass is not (as in these two genera) 

 connected with the stalk, but it is attached to the mantle at some dis- 

 tance from the stalk. I unite here the species which show the above- 

 mentioned characteristics under the new generic name Loxothylacus. 

 Accordingly, I consider the family Sacculinidae composed of four 

 genera with the following chief characteristics : 



Sacculiiia.: Mesentery complete, testes straight, visceral mass united 

 with the stalk. 



Loxothylacus: Mesentery complete, testes curved, visceral mass 

 united with the mantle at some distance from the stalk. 



Drepanorchis : Mesentery incomplete, testes curved, visceral mass 

 united with the stalk. 



Heterosacaus: Mesentery almost wanting (confined to the imme- 

 diate vicinity of the stalk) . 



There are a great number of species belonging to the Sacculinidae, 

 although this number is smaller than Giard (1887, 1888) assumed, 

 who upheld that every species of crab had its own species of rhizo- 

 cephalous parasite. This theory (the theory of the absolute specific- 

 ity of the parasites) has been combated by Smith (1906), who, how- 

 ever, went to the other extreme and expressed as his opinion that all 

 the parasites of the genus Sacculina belong to one and the same valid 

 species, S. carcini Thompson. Smith based this conclusion on the 

 fact that he found no morphological differences between the para- 

 sites on diflPerent crabs in the material at his disposal. Now, this 

 conclusion also was too far-reaching, for Giard (1887) had 

 proved already that the parasite called by him Saecvlina fraissei 

 {= DrepanoTchis neglecta (Fraisse), see Boschma, 1927) differs 

 from S. carcini in constant morphological characteristics, the 

 form and situation of the testes in both species being very different. 

 Moreover, Kossmann (1872) had described morphological differences 

 between species of Sacculinidae which were found on different hosts. 

 His diagnoses of new species were based mainly on the structure of 

 the chitinous covering of the mantle, which in many cases bears 

 definitely shaped small excrescences. Kossmann also was convinced 

 that a certain species of Sacculina could live on one species of host 

 only and accordingly described too many forms as new species. The 

 discovery of the appendages of the cuticle, however, was a valuable 

 progress in the study of the Sacculinidae as it furnishes reliable 

 data for the taxonomy of the group. Especially in tropical 

 species (Kossmann's material had been collected at the Philippine 

 Islands) these small projecting parts of the external cuticle are dis- 

 tinctly visible. In the same way in the collection of the Siboga 



