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



simple longitudinal expansions of tlie basement-tissue, which stain more deeply than the 

 basement-tissue itself. They appear at first like a series of V-shaped processes along its 

 inner border. The limbs of the V gradually separate and enclose a canal which traverses 

 the whole region of the basement-tissue (that is, extends from its outer to its inner border). 

 Proceeding distally (anteriorly), the chitinous skeleton becomes much more complex, 

 from the fact that it develops tubes for the transmission of the circulatory fluid, extends 

 round the whole circumference of the body, except a limited region at the neural hiatus, 

 and, moreover, splits into two or more rows. In the formation of the circulatory 

 channels, the chitinous longitudinal processes would first seem to form a thin arch with 

 two dilated pillars, and then a complete ring with much thinner walls. Before complete 

 separation of the basal folds of the web occurs, the sections show a double row of these 

 channels, one being on the outer edge of the volution and the other on the inner — a 

 basement-layer separating the former row and its spongy vascular tissue from the latter. 

 A line intersecting the coils would thus (before separation occurs) pass through four 

 complete series of these channels, and in certain positions (involving the central volu- 

 tion) through six. At the base the inner row at the margin of the central coil is 

 incomplete, but by and by the vessels form a continuous series round the edge. The 

 structure of one of these double basal rows is as follows : — Externally is the cuticular 

 layer (PI. III. fig. 3), which, however, in the preparations does not seem to be well 

 differentiated, since it forms only a definite boundary to the hypoderm. This feature 

 is probably related to the branchial functions of the region. Within is a layer of 

 hypoderm, which varies in thickness and colour according to the level of the section. 

 Its structure agrees with that of the body-wall, and accords with the same tissue in 

 the Annelids and Nemerteans. Directly under it are the elastic arches of the outer row 

 of channels, which are incomplete internally, — each debouching into a large vascular 

 space, which with connective tissue fills up the area between the outer row and the line 

 of basement-tissue separating the two series. In transverse section the basement-tissue 

 shows the thickened sides formerly alluded to, and is often finely streaked, but the 

 latter is probably only an optical peculiarity. Within the line of basement-tissue are 

 the inner series of channels, which have complete walls (and the outer row would 

 thus seem to be less developed at a given level than the inner) ; and in transverse 

 section present a noteworthy uniformity in appearance, viz. with a dumb-bell shaped 

 outline of the inner wall, from the thickening of the median region on each side, while 

 the lines of the outer wall laterally are nearly parallel. The outer and inner arches of 

 the vessel are thinner. Only a slight quantity of connective-tissue separates each 

 chitinous channel from the septum or line of basement-tissue, while the other arch has 

 a thick coat of hypoderm, which soon (as we proceed distally) becomes grouped in a 

 series of fan-shaped masses. The inner surffice of the channels (of basement-tissue) is 

 furnished with an epithelial lining ; and since in section the arch next the septum 



