LTJCERNARI^ AND THEIR ALLIES. 31 



tions (47, 48). The peculiar relations of this stratum are best displayed by two 

 sections made at right angles to each other, the one {f,gs. 47, 47", b) passing from 

 the proboscis lengthwise along a partition, so as to split it, and the anchor which 

 lies opposite to it, and the other cut traversing it crosswise {figs. 61, 63, h, ?/) so as 

 to show its breadth between the two adjoining umbellar camertc. In the longi- 

 tudinal section {figs. 47, 47*, h) it exhibits a pretty uniform thickness from the 

 proximal to the distal end of the partitions ; but is not so thin by one-half as at 

 intermediate points, as the crosswise section {figs. 61, 63, ?>*) shows very conclu- 

 sively; and it evidently constitutes almost all — a thickening (»/') of the opsomyo- 

 plax along this line occupying the rest — of the depth and breadth of the partition, 

 the chondrophys (c^) which meets it, scarcely projecting beyond the level of the 

 posterior inner surface of the adjacent camerse. Just before it reaches the distal 

 end of the partitions it begins to thicken, and finally that part which fills these 

 partitions terminates abruptly {fig. 47, h) at the passage-ways (v") between the 

 compartments of the main cavity, while its more anterior portion stretches onward 

 into the anchors {Ir), and is there disposed and terminated in the same way as 

 indicated when speaking of this layer in the four alternate anchors. At the proxi- 

 mal ends {fig. 47", ■^^) of the partitions it runs backward behind the base (p^) of 

 the proboscis a short distance, and forms a part of the four low ridges, which were 

 described in a former paragraph (53) upon the posterior division of the main 

 umbellar cavity, and thins out to nothing just at the point Avhere the peduncular- 

 muscle (r^), in passing forward to the anterior parietes of the umbella, strikes the 

 lining wall (*) of this cavity. Here too the chondromyoplax is perforated, or rather, 

 since it is scarcely wider than the muscular cord, is cut in two by it, as the latter 

 penetrates to the front and joins the anterior subdivision of the muscular system — 

 the opsomyoplax {m*). 



68. One of the most convenient methods of getting a general view of the varying 

 thickness and irregularities of the chondromyoplax, is by taking advantage of the 

 sometimes unusual elongation of the corners of the umbella, and making sections 

 across them singly or across a pair of them, as we have done {figs. 61, 62). In the 

 one across a single corner {fig. 62) we see that where the layer in question comes 

 to the edge of the umbella it has a very abrupt, truncate-concave termination {b), 

 fitting, with the intervening opsomyoplax (/.-'), against the convex abrupt edge of 

 the chondrophys (c), like a ball-and-socket joint. We notice, too, that near these 

 edges it is deeply indented by folds {m^) of the marginal muscle, and that the same 

 phenomenon occurs over a narrow space close to the genital saccules, where the 

 muscular bands {m') are strongest and heaviest as they trend parallelwise to the 

 partitions, in their course toward the periphery of the umbella. Its relation to the 

 genital saccules, we have already (^ 66) pointed out, and we will, therefore, proceed 

 to consider the other section {fig. 61), which in this case includes one of the par- 

 titions (\/-^). The marginal termination, and the relation of the chondromyoplax to 

 the genital saccules is the same as in the previous sectional view, but between the 

 saccules and the partition it dificrs in that the whole breadth of it is strongly 

 indented by thick folds (??;) of the opsomyoplax, and immediately opposite the par- 

 tition this muscular layer (;/;') is so thick as to reach almost to the base level of the 



