175 
They all show the æcological habit, for Polyclads very peculiar, of 
living on mud bottom, a reason perhaps why these forms of ancient 
organization survive in these regions. Doubtless their male appa- 
ratus is more primitive than that of Stylochidae. With increased 
knowledge of Stylochidae the gap between these forms and Stylo- 
chids is steadily diminished. New Polyclads from my South Sea 
journey complete in a desirable way our knowledge of the rela- 
tionship of Plehniidae to Stylochidae. 
Since Meixner’s table of the geographical distribution of the 
Stylochidae seems rather antiquated now, as the number of recog- 
nized species of Stylochidae has augmented since its publication to 
considerably more than double, not to speak of the number of 
genera, I think it necessary to give a new table. I have arranged 
the many Stylochus-species according to occurrence. For sake of 
convenience I have enumerated the genera in the same order as 
they occur in my determination table. 
Briefly summarizing the table, one finds it rather aston- 
ishing that not less than 35 Stylochids are described from the Indo- 
Pacific area, while in the Atlantic (the Mediterranean Sea and Black 
Sea inclusive) there are not more than 11 species known. After 
the description of my half a dozen new Stylochids from the Paci¬ 
fic, the difference will be yet more pronounced. A study of the 
table will demonstrate that all the Atlantic species but one, Idio- 
planoides atlanticum Bock, belong to the genus Stylochus. Of these 
ten Atlantic species, Europe and the United States have equal lots. 
I shall then direct the attention to the circumstance that none of 
them is common for both areas, a case perhaps worthy of renewed 
investigations. Fully unbelievable or unexpected the case is not, 
since, as a rule, the Stylochids seem to have a rather'limited di¬ 
stribution. This has already been pointed out by Meixner, and ac¬ 
cording to the same esteemed author, is partly due to the faet that 
the development, as in St. neapolitaniis, is direct. But he also calls 
attention to the faet that Goette and Lang have recorded a deve¬ 
lopment with pelagic stages in St. pilidium. During my embryolo- 
gical researches carried out in Japan, I found the same valid for 
another Stylochus species.'®') 
*) This species I shall call ferox, as it is a voracious animal, appearing as 
a destroyer of young oysters. 
