98 THE VOYAGE OF H.M.S. CHALLENGEE. 



below with the plexiform ghmd rising alongside the fore-gut. But it is very marked 

 indeed in transverse sections behind the mouth (i.e., between it and the anus), as is well 

 shown in the case of Pentacrinus loyville-thomsoni (PI. LVII. fig. 1, Ip), Antedon 

 eschrichti (PI. 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. It is 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 

 (PI. LVII. fig. 3 ; PI. 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 PI. 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 

 PI. 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 traverse? the anterior ambulacrum. The section figured (PL LIX. fig. 5) passes 

 through the left angle of the peristome, from which the two lateral amljulacra diverge ; 

 and the labial plexus is seen as a broad band {Ip), which lies between the water-tubes 

 depending from the water-vascular ring {ivt), and the fore-gut {/(j) ; 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 dichroie pigment which so unpleasantly increases the difliculty of accurate 

 observation in the Naples variety of Antedon rosacea. 



In Promachocrinus herguelensis, 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 difiers 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 PI. LX. 

 figs. 3, 5, so. The former represents a longitudinal, and the latter a transverse section 

 of it ; while in PI. LIX. fig. 8, a portion of the spongy organ of Promachocrinus 

 hergiielensis 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 



