XIV 
INTRODUCTION. 
and stretching vertically through the cavity inclosed by 
the column. Each principal septum (Plate XI. fig. 1, a), 
in any of the normal species, is inserted, by its outer edge, 
into the column-wall throughout its entire height ; by its 
lower edge, into the base, from the wall to the centre ; by 
its upper edge, into the disk, from the margin to the mouth ; 
and, by its inner edge, into the stomach, from the lip, almost 
to the free bottom of that viscus. From thence the inner edge 
recedes with an arching outline, and is free, until it is 
gradually merged in the lower edge at the centre of the 
base. Between these primary septa, others are developed 
in succession, partitioning off the imperfect chambers thus 
formed. But the septa of each successive cycle, while still 
inserted in the column-wall throughout, spring from the 
stomach at higher and higher points, and terminate at 
points more and more remote from the centre of the base. 
The number of septa depends, to a certain limit, on the age 
of the individual, but in Peachia it never exceeds twelve, 
and in Halcampa microps, eight. 
In Peachia, the tissue of the septa is very dense, and 
still more so in T. crassicornis, where it assumes a firmness 
almost cartilaginous, and a decided blue colour. 
The muscular tissue of the disk protrudes in the form 
of hollow cones, which are the tentacles : each of these 
springs from an interseptal chamber, and hence their deve- 
lopment is in cycles corresponding to that of the septa. 
The fibres which compose their walls are very delicate. 
3. Nervous and Sensory System. I have been as unsuc- 
cessful as my predecessors, in my search for nervous threads 
or ganglia ; still, I have little doubt that such exist. I 
should expect their presence in the form of a ring, sur- 
rounding the mouth, perhaps with a pair of ganglia at the 
gonidial tubercles, distributing threads to the tentacles. 
I have never observed any trace of auditory vesicles or 
otolithes, nor any organs tliat I could regard as eyes ; not 
even in the rudimentary form of those aggi’egations of pig- 
ment-cells, that occur on the margin of the Xaked-eyed 
Medusae. A delicate sense of touch certainly exists, dis- 
tributed over the entire smlace, but specially localized in 
the lips and the tentacles. The occasional elongation of 
one or more of these latter organs, and their employment 
(as described at pp. 34 — 36, infra) , indicate the existence 
of an active tactile faculty, and not merely of passive 
