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



parts are very differently constituted. The former, which is the only part externally visible, 

 is covered with transverse ■wrinkles, crossed here and there with longitudinal furrows. 

 It terminates above in twelve knobs which are placed like a corona round the entrance 

 to the orifice of invagination, and lie close together so as to greatly contract the opening. 



In order to vmderstand the construction of the upper part, it is necessary to open up 

 the animal longitudinally, then we see that it extends about 1 cm. inside the animal ; it 

 is covered with twelve strongly projecting sharp-edged longitudinal ridges, which begin 

 at the twelve knobs, become higher as they run downwards (upwards in the natural 

 position), till each of them ends in a nose-like projection. This projection is again 

 divided by a longitudinal furrow into a larger and a smaller process. The ridges and 

 their bifurcated ends are extremely smooth, very soft, and of a whitish colour. They 

 consist, however, only of connective tissue, like the rest of the wall. 



The varied aspect of the surface of the body is caused by the varying character of the 

 epithelium ; on the invaginated part of the wall (PL VI. fig. 5) it is a ciliated cylindrical 

 epithelium, such as we find in most Actiniae ; in the other parts it is without cilia, but 

 instead of cilia is covered with a deposit, which may be divided into two layers. One of 

 these (PL VIII. fig. 1) is an irregular, fibrous cuticle, which is stained an intense red by 

 carmine, the other is a mucous layer permeated by foreign bodies, Ipng outside the cuticle. 



The mesoderm consists of extremely fine fibrillee which cross one another in all 

 directions, so that it appears as a finely granulated mass in transverse section. It is 

 partially laid in strata parallel to the surface ; in it there are small roundish concrements, 

 which are strongly coloured by carmine, and the structure of which recalls that of granules 

 of starch ; they are made up of indistinct concentric layers, frequently appear in section 

 like a figure 8) and are limited to the superficial layer of the mesoderm. 



The existence of a strong circular muscle, which is indeed easily discovered, might be 

 inferred merely from the high degree of contraction. It is mesodermal, and the chief bulk 

 of it lies in the invaginated section of the wall, where it begins close to the commence- 

 ment of the oral disk, or, to speak more accurately, to the origin of the tentacles (PL VI. 

 fig. 5) ; it extends a considerable way into the outer section of the wall, into which it 

 gradually passes. It is separated from the endoderm by a broad layer of connective 

 substance, so that it lies nearer to the ectoderm than to its place of origin, and consists 

 of numerous very small bundles of fibrillae grouped together into bundles of the second 

 order (PL VIII. fig. 10). The latter are usually flattened, and in transverse section show 

 bands lying perpendicular to the surface of the endoderm. 



The comportment of the oral disk is the same as that formerly described in Calliactis 

 parasitica. The muscular fibres are still chiefly ectodermal, and the lamella is not 

 thickly pleated, though at the same time single fibrillse and groups of fibriUaj have 

 passed into the mesoderm. The boundary line between the mesoderm and the ectoderm 

 is therefore indistinct as in Calliactis. 



