520 T. A. STEPHENSON 
main feature six pairs of macrocnemes; but there are varia- 
tions ; the full six pairs may not be developed (Pentactinia 
and some individuals of Halecampa), or there may be an 
extra couple (Scytophorus). Microcnemes may be present 
or not. 
This idea regards the genera Halecampa and Halcam- 
poides as constituting, jointly, types of the family, and no 
separation of these on account of sphincter is wise. It brings 
in Pentactinia and Scytophorus, the one as a slightly 
under-developed, the other as a slightly over-developed, 
Halcampa-form. Indeed, these two are very like Hal- 
campas but for mesenterial oddities slightly deviating from 
type. <A case parallel to that of Pentactinia is that of 
Decaphellia, a Phellud with subnormal number of macro- 
cnemes. ‘T'ext-fig. 8 shows a transverse section of a Halcampid 
for contrast with that of one of the Ilyanthidae in the strict 
sense as described in the next paragraph and illustrated in 
Text-fig. 9.4 
If we now take the genera Ilyanthus (mitchell), 
Harenactis, Hloactis, Peachia, and Haloclava, 
we find a rather different type of structure. The mesenteries 
are never fewer than ten pairs in adult animals, and vary 
up to about eighteen pairs. They all have virtually the grade 
of macrocnemes, even though there may be differentiation 
among them—except that in Peachia some of them are 
devoid of filament and gonad, but have strong retractors and 
are not microcnemes. Tor the rest they often attain fair size 
and may have stout bodies (capable of becoming vermiform) 
or very long ones. Suckers present or absent. Cinclides may 
occur. ‘Tentacles simple or capitate, eight, twelve, twenty, or 
more, up to about forty. Little or no sphincter. There may 
be only one siphonoglyphe, which in Peachia is specialized 
into a conchula. In Peachia we have six perfect pairs of 
mesenteries (or rarely fewer?) and four secondary pairs; in 
1 In this figure the gaps in the mesenteries are due to the fact that 
the section passes through the region of mesenterial stomata—in most 
regions the mesenteries would be continuous. 
