98 THE VOYAGE OF H.M.S. CHALLENGER. 
below with the plexiform gland rising alongside the fore-gut. But it is very marked 
indeed in transverse sections behind the mouth (7.e., between it and the anus), as is well 
shown in the case of Pentacrinus wyville-thomsoni (Pl. LVII. fig. 1, Up), Antedon 
eschrichti (Pl. LX. fig. 5), or of any Actinometra. It gradually diminishes in size as 
the distance from the mouth increases, occupying an intermediate position between the 
two posterior ambulacra. Itis usually rather nearer to the left one (C), but is sometimes 
closer to the other (D). The genital vessels of these ambulacra are derived from it 
(Pl. LVIL. fig. 3; Pl. LX. figs. 1, 2—gv), and it finally passes insensibly into the inter- 
visceral vesssels of the hinder part of the disk. In the specimen of Antedon carinata, 
one section of which is represented in Pl. LX. fig. 2, the labial plexus lies, as usual, 
rather nearer the left posterior ambulacrum, continuing very close and compact until just 
in front of the anal tube, where its meshes open out, and it passes into the ordinary net- 
work of intervisceral vessels. 
In like manner the examination of a series of longitudinal sections shows that the 
labial plexus is denser, and extends farther from the peristome on the left side than it 
does on the right. In the specimen of Antedon rosacea, a section of which is figured in 
Pl. LIX. fig. 5, the plexus is much more developed, both anteriorly and posteriorly, at 
the left edge of the mouth-slit than at the right edge, or even than in the median plane 
which traverses the anterior ambulacrum. ‘The section figured (Pl. LIX. fig. 5) passes 
through the left angle of the peristome, from which the two lateral ambulacra diverge ; 
and the labial plexus is seen as a broad band (/p), which lies between the water-tubes 
depending from the water-vascular ring (wt), and the fore-gut (fy); it diminishes in size 
as the distance from the mouth increases, and loses its individuality when the two 
ambulacra become differentiated. 
Unfortunately I do not know the locality of this specimen; but it is singularly 
devoid of the dichroic pigment which so unpleasantly increases the difficulty of accurate 
observation in the Naples variety of Antedon rosacea. 
In Promachocrinus kerguelensis, in Antedon eschrichti and its allied species Antedon 
quadrata and Antedon antarctica, a portion of the labial plexus between the mouth and 
the anal tube differs very considerably in structure from the rest of this organ. The 
limits of this portion are so well defined, and it differs so much from the remainder of 
the labial plexus, that I propose to designate it by the name “spongy organ.” Its 
relations to the rest of the labial plexus in Antedon eschrichti are shown in Pl. LX. 
figs. 3, 5, so. The former represents a longitudinal, and the latter a transverse section 
of it; while in Pl. LIX. fig. 8, a portion of the spongy organ of Promachocrinus 
kerguelensis is shown more highly magnified. It lies between the mouth and anus on 
the left or eastern side of the gullet, and therefore is slightly nearer to the left posterior 
than to the right posterior ambulacrum. 
In its most fully developed condition, only found between the mouth and anus, the 
