Gatty Marine Laboratory, St. Andrews. 11 



between them. The cuticle and hypoderm externally become 

 crenate and then notched, whilst spaces or slits appear 

 between the chordoid ovals, by-and-by pass to the surface, 

 and thus truncated fillets representing the separate filaments 

 are formed all round the edge of the branchial base. The 

 outer edge of each has a thick coat of hypoderm under the 

 cuticle, but this diminishes internally on the sides, becoming 

 thinner in its progress inward, the whole area resembling a 

 narrow wedge with the broad end outside (PL II. fig. 12). 

 Within the broad end is the basement-membrane and a 

 " perichondria! " area surrounding the chordoid oval from 

 which the median strand passes inward to support the blood- 

 vessel. In this region the bases of the filaments and their 

 axes are joined by a long band of the " perichondria! " 

 substance, the appearance after partial maceration resem- 

 bling a chain of Perophora listeri or similar series of tunicate 

 stolons. 



The two bands of muscle then show signs of diminution. 

 Just before the filaments separate, small clear spaces occur 

 at somewhat regular intervals in the interfilamentar tissue, 

 but they are not visible after separation. At this level the 

 sections of the bases, of the filaments have their longest 

 diameter radial (PI. II. fig. 12), but this by-and-by shortens, 

 and their inner border separates from the internal lining 

 at the base, and each forms an independent filament, the 

 muscular fibres, meanwhile, gradually diminishing. The 

 chordoid cells in these form a double row (PI. II. fig. 12), 

 sometimes with two nuclei, but generally with a single 

 nucleus in each, and the number of cells diminishes in the 

 distal parts of the filament (PI. I. fig. 4). When a pinna 

 is cut longitudinally, a double row of cells is present in the 

 sections (PI. I. figs. 5 & 6), besides the external investment, 

 or, as the knife slants superficially, the closer lines indicating 

 the cells of the hypoderm intrude, as at the lower part of 

 the drawing (fig. 6). The nerve occupies an area near the 

 ciliated groove at the inner border. The double character 

 of the slits is still preserved, for one-half of the inner joins 

 that of its neighbour to the right, and the other that to the 

 left. Then the diameter of each filament, now free, still 

 further diminishes, and the blood-vessel is separated from 

 the chordoid skeleton only by a narrow belt of connective 

 tis>ue. Moreover, a double row of pinnee springs from the 

 inner and narrower edge, the outer having its thicker belt 

 of hypoderm and its more massive connective-tissue layer and 

 nerve internally. A single row of chordoid cells passes from 

 the chordoid oval into each pinna as its skeletogenous rod, 

 and thus the whole system is continuous from its massive 



