32 
THE VOYAGE OF H.M.S. CHALLENGER. 
Family PERISIPHONIMl. 
Character of the Family. Trophosome formed of two essential constituents — an 
axial and a peripheral, the axial consisting of a single tube which carries the hydrothecse 
and the peripheral, formed of numerous tubes destitute of hydrothecse, and united into a 
fascicle which surrounds the axial to a greater or less extent, and allows the hydrothecse 
to project through intervals between its component tubes. Hydrothecae never adnate by 
their sides to the axial tube where this is covered by the peripheral. Peripheral and 
axial tubes not inseparably coalesced. 
There is among the Calyptoblastic Hydroids no more natural and distinctly defined 
family than that of the Perisiphonidae. The remarkable structure of the trophosome 
with its axial hydrotheca-bearing tube enveloped by the peripheral fascicle is, except 
in Grammaria, quite unknown in any other group; for this condition must not be 
confounded with the fasciculation of the stem which occurs in many Plumularinse and is 
common in Halecium, Sertularia, Thuiaria, and other genera (see Part I. p. 4), in 
which the component tubes are not divisible into an axial tube which carries the hydro- 
thecse and peripheral tubes which are destitute of hydrothecse. Except in Cryptolaria 
the gonosome of the Perisiphonidae is entirely unknown. 
Four genera, all of which occur among the dredgings of the Challenger, must be 
assigned to the family Perisiphonidae. The essential points of difference between these 
are given in the following scheme : — 
Axial tube not wholly enveloped by the peripheral. Hydrothecse ' 
sessile or pedunculate, never adnate to the axial tube, and with 
the cavity directly continuous with that of the stem or peduncle. 
No sarcothecse present. 
> Lafoea. 
Perisiphonid-E. 
i 
Axial tube not wholly enveloped by the peripheral. Hydrothecse 
pedunculate, never adnate to the axial tube, and with the cavity 
distinctly differentiated from that of the peduncle. No sarcothecse 
present. 
>- Lictorella. 
Axial tube wholly enveloped by the peripheral. Hydrothecse pedun- 
culate, distichous. Sarcothecse developed on the stem. 
| Perisiphonia. 
Axial tube not wholly enveloped by the peripheral. Hydrothecse 
sessile on the axial tube, to the free portion of which they are 
more or less adnate; distichous or scattered. No sarcothecse 
present. 
> Cryptolaria. 
Lafoea , Lamouroux. 
Lafoea, Lamouroux, Expos. Method., 1821. 
Generic Character. Trophosome. — Hydrocaulus in its main stem and principal 
branches consisting of a single axial tube, enveloped by numerous peripheral tubes, but 
