521 
1. Longitudinal. 
2 . Superficial circular. 
3. Deep circular. 
4. Gastro-parietal. 
In the gastro-parietal set, especially, which extend from the walls 
of the stomach towards the external surface of the body, he had no 
difficulty in demonstrating a true tubular structure. Under stimu- 
lation they contract longitudinally, and become thickened trans- 
versely ; and then they are plainly seen to be thick-walled tubes, 
whose cavity encloses a granular matter. The tubules composing 
the other three sets seem to be also contractile, though this property 
is not so apparent in them as in the gastro-parietal set. All the 
four sets depolarise light, and present a similar appearance under 
the polariscope. 
To the two oval ciliated spaces, situated one at either side of the 
ocellus, the author gave the name of tentacular discs. The margin 
of each disc is marked by a double line, on whose interval is borne 
a series of short hollow tentacular processes, simple in young speci- 
mens, but irregularly lobed or ramified in adults, covered over their 
whole surface with fine actively-vibrating cilia, and containing nume- 
rous pigment cells in their walls. The double line marks the boun- 
dary of a canal which runs round the disc upon its deep surface, and 
into which the bases of the tentacles open, the canal ceasing to be 
continuous at the inner extremity of the major axis of the disc, 
where there is a short space destitute of tentacles. The author main- 
tains that the cavity of the tentacles, and their common basal canal, 
communicate, through the bifurcations of the funnel, with the general 
gastro- vascular system. 
Eight chord-like organs have been noticed by observers, and have 
been described in detail by Agassiz as extending from beneath the 
ocelliform body to the extremities of the eight meridional bands ; 
but the fact has not been hitherto noticed, that where they lie upon 
the cushion-like body which supports the sense capsule, they coalesce 
two and two, so as here to be but four in number. These four chords 
are clothed with true vibratile cilia. The author also described two 
additional chords, which originate also beneath the ocellus, one at 
each side, exactly opposite to the tentacular discs, and thence running 
outwards without dividing, lose themselves in the discs. These, 
like the former, are clothed with vibratile cilia. The true origin of 
the chords he believes to be in a ganglion-like tubercle which lies 
immediately under the sense capsule, and between it and the cushion- 
