REPORT ON THE TUNICATA. 97 



sac bulges to a greater or Less extenl into the interior of the cavity, so as to increase the 

 area of the wall, and along which the internal longitudinal bars are present in greater 

 number, and are therefore more closely placed than in other parts of the sac (PI. VIII. 

 fig. 3). In an average sixe of fold there are ten internal longitudinal bars, counting both 

 sides of the fold, while, in the plain space between two folds, there an- only four internal 

 longitudinal bars. Yet that open space is nearly as wide as the fold would be if spread out, 

 that is to say, it is nearly twice as wide as the fold in its normal doubled-up condition. 

 In fact, on an average, the internal longitudinal bars are about twice as closely placed 

 on the folds as they are elsewhere. They are not, however, placed at regular intervals, 

 but become more and more closely placed as they approach the projecting edge or crest of 

 the fold ; while the first bar of the fold shows but little difference from the arrangement 

 between the folds, the fourth, fifth, and sixth are so closely placed that the meshes are 

 "reduced in places to mere chinks, and are in some cases obliterated (PL VIII. fig. 3, 

 br.f). 



A conspicuous feature in this branchial sac is the presence of spicules in the interior 

 of the vessels (PL VIII. fig. 3, sp.). They lie in the inner part of the wall of the vessel. 

 and are present in greatest number in the internal longitudinal bars, and especially near 

 their points of intersection with the larger transverse vessels into which the spicules 

 sometime- extend. I have never seen them in the smaller transverse vessels. 



These spicules are composed of carbonate of lime, and are often of very considerable 

 size, up to •;> mm. in their greatest extent. There seems to be no prevalent form or plan 

 of growth for them, though they have a characteristic appearance, as they are generallv 

 slender and branching, and the outline is formed of gentle curves, there being no sharp 

 points or angles. The smallest and simplest forms noticed were minute fusiform spieula ; 

 these, when a little larger, began to have their outlines somewhat wavy, and frequently 

 one of tin' ends was forked, or a slight branch had made its appearance near the middle. 

 From this all stages of complication may lie found up to the largest forms which are 

 often considerably branched. 



On examining these spicules closely with a high power, one notices that they are 

 invariably marked by a series of delicate lines, of which the outer ones run exactly 

 parallel to the outline of the spicule, following all its curves, while the more internally 

 placed lines do so to a less degree, and finally the series ends near the centre of the 

 spicule in a few concentric curves, the whole having the closest possible resemblance to a 

 system of contour lines on a. map (PL VIII. fig. 6). These markings no doubl 

 represent stages in the deposition of the carbonate of lime. In some of the spicules, if 

 uo1 in all of them, there exists a central cavity, which may be prolonged along the 

 branches to a considerable extent as fine canals, along which air and staining fluid- 

 were observed to pass in some of the broken spieula. 



The wall of the vessels of the branchial sac is composed externally of a layer of thin 



