8 THE VOYAGE OF H.M.S. CHALLENGER. 



basement-tissue (bt) of considerable thickness. This basement-tissue anteriorly forms the 

 skeletogenous layer in the tentacles, — as described in the preliminary account, — and at 

 the base of these it first appears in mass on the neural side of the inner whorl, 

 beneath the pigmented hypoderm of the part. Moreover, considerable thickening of 

 the basement-tissue occurs towards the central region of the fans ; indeed, it forms a 

 strong ridge to which their muscles are attached. The basement-tissue gradually 

 extends round the whorl as we proceed backward, but it becomes less massive in the 

 region first mentioned, viz. on the neural convexity of the inner whorl. A little 

 further backward the tissue has also largely increased on the inner free margin of the 

 whorl, and the hj'podermic pigment from the former region is continuous all round the 

 whorl to the posterior projecting fold with the furrow to its outer side. This thickening 

 of the basement-tissue soon supports the great nervous mass which is superficial to it, 

 and an increase is also apparent in the furrow on the posterior aspect ; moreover, it 

 generally stains more deeply than the hypodermic nerve-tissue. It thus sjjecially 

 supports the nerve-centre and posterior short trunks. Then it forms a massive central 

 pillar for the muscles of the base of the fans, stretches all round the bases of the double 

 row of branchial channels, the thickest parts occurring in the inner row laterally and 

 posterior to the oral region (PI. III. fig. 1, in longitudinal section). 



When the anus enters into the line of section, and the nephridia have become 

 quite posterior in position so as to project externally, this skeletogenous tissue presents 

 a complex aiTangement, its main masses being in the centre of the lateral fans, and 

 supporting the vascular lacunae and the various canals of the region (PL II. fig. l). 

 With the termination of the nephridial canals the process of basement-tissue on the 

 inner side ceases, while the outer trends away to the blood-channel on tlie lateral wall 

 of the body, and finally merges into the special layer in the latter on the disajjpearance 

 of the space. On the whole, the skeletogenous tissue of the anterior region is much 

 more largely developed in the adult than in the young examples. 



Behind the tentacular region the basement-tissue forms a layer all round the 

 body, beneath the hypoderm, and sends processes inward ventrally to form the 

 mesenteries enclosing the rectum. Thereafter the chief feature of note is the thicker 

 layer of the tissue on each lateral arch, the central (or neural) having a thinner layer, a 

 disposition which partly accounts for its prominent condition. As, however, the layer 

 becomes of uniform thickness posteriorly, it would appear that the attachments of the 

 mesenteries and radial muscles are also concerned in these modifications of the body- 

 Avall. The density of this layer anteriorly probably gives the region its characteristic 

 rigidity. Posteriorly it becomes considerably thinner, especially in the terminal 

 dilatation, though just at the tip it again assumes greater bulk, and supports the 

 peculiar glandular organ of the region. 



This basement-tissue thus performs all the functions of an internal skeleton for 



