REPORT ON THE TUNICATA. 95 



Outside this normal test substance, and therefore winding between the canals, air 

 masses and trabecule of a bright yellow-brown tissue (PI. IX. figs. 1 ami 2, t.m.v.) con- 

 centrically laminated — appearing as longitudinal striatum in longitudinal sections of the 

 bars. An examination of this tissue shows that it is merely a modification of the 

 neighbouring test substance caused by some sort of cornification taking place in successive 

 layers so as to produce the concentric lamination. The bright yellowish-brown matrix is 

 closely nbrillated, the bundles of fibrillar running as a rule parallel to the concentric 

 laminae. In this fibrous matrix numerous protoplasts are imbedded; they are very 

 minute, circular to fusiform in outline, and have comparatively large, brightly refracting, 

 circular nuclei. 



The blood-vessels occupying the canals (PI. IX. fig. 1, ped. c.) and surrounded by the 

 unmodified test substance are of various sizes. A large trunk, and occasionally one or 

 more small branches from it, are found in the central large canal, while smaller vessels 

 occupy the other spaces ; like the canals they lie in, these vessels intercommunicate — 

 they form a branching and anastomosing system. The wall of all these vessels consists 

 of a single layer of cells. These are large and elongated, varying in shape from fusiform 

 to oblong, and have large and distinct circular and centrally placed nuclei, and finely 

 granular protoplasm. They lie with their long axes parallel to the length of the vessel, 

 and in transverse sections appear as small round cells. 



TJie Mantle is thin but moderately strong ; it still (in spirit specimens) adheres to 

 the inner surface of the test, but the connection is slight, as it may be peeled off with 

 ease. The muscle bands are strong but distantly placed, so as to form an open network. 

 Most of the bands run transversely to the longitudinal axis drawn through the point of 

 attachment to the peduncle and the opposite extremity of the body, but a few longitudinal 

 bands are also present, radiating from the branchial and atrial apertures ; these lie 

 internally to the transverse muscles. On the branchial and atrial siphons the muscle 

 bands lie transversely, are more regular and parallel than elsewhere, and considerably 

 closer ; these can hardly be characterised, however, as sphincter muscles. 



The muscle bands are flattened like ribbons, and contain on an average about fifty 

 fibres in their breadth. The fibres are rather large, of a much elongated fusiform shape, 

 and are closely packed in bundles. They stain deeply with carmine, but no distinct 

 nuclei are visible. In teased bundles, however, some rather smaller fibres (or series of 

 fibres) were noticed having swellings at intervals, or shaped like a series of spindles 

 joined by their ends ; these had distinct circular nuclei in the wider parts. No transverse 

 stripes were observed in any of the fibres. 



The greater part of the mantle is composed of connective tissue, which envelopes and 

 stretches between the muscle bands. It is in the form of a thin layer of gelatigenous 

 areolar connective tissue — -a delicate transparent matrix, in many places apparently 

 structureless, in others finely fibrillated, and sometimes formed of bundles of white 



