REPORT ON THE TUNICATA. 99 



together ; and that the brown colour of the bottom of the groove is caused by the granular 

 brown cell elements in that locality. The spicules are composed of carbonate of lime, 

 and an- very similar to those found in the vessels of the branchial sac, but here they are 

 larger and considerably more branched. In the prominent while edge of the endostyle 

 (PI. VIII. fig. 5, w.e.) each of the quadrangular patches is a dense mass of spicules 

 closely matted together in the centre, and rather more open at the edges where the tips 

 of the branches are seen projecting. The different patches are united together by the 

 branches which stretch from one to the other. The rather denser alternate patches 

 send down branches of spicules into the translucent area (PI. VIII. fig. 5), while the 

 intermediate patches, which are not so dense, have no connection with that area. 



The translucent area, immediately inside the prominent white edge, has a series of 

 spicules stretching longitudinally along its centre and leaving a clear space along each 

 side (PI. VIII. fig. 5, t.a.). Some of these spicules are very complicated, extending for a 

 great distance, and branching and uniting again so as to form a tangled mass. They are 

 united to the spicules of the outer white edge by the branches which dip in from the 

 denser patches, but have no connection with the spicules of the central brown area. 



When the lateral brown bands encroach upon the translucent area they lie along the 

 line of spicules, so as to leave a clear space separating them from the central brown area 

 internally and from the white edge externally. The central brown area is also provided 

 with spicules (PI. VIII. fig. 5, c.b.a.), but here they are not nearly so prominent on account 

 of the strong colour of the cells overlying them. The spicules are arranged in two longi- 

 tudinal series leaving a clearer space in the middle, across which however they send 

 branches which unite here and there. They extend laterally slightly beyond the brown 

 area into the clear band on the inner side of the translucent area, and may even overlap 

 the tips of the spicules of the translucent area, but they were never observed to unite 

 with them. 



Over the white prominent edge and the translucent area the epithelium forming the 

 surface of the endostyle is clear and transparent. The cell elements, however, are distinctly 

 visible. In surface view they are square or polygonal cells of moderate size provided 

 with distinct dark nuclei. When seen in profile they are cubical. The lateral dark 

 brown bands are formed of columnar epithelium. The cells are very long and narrow, 

 and closely packed together. They taper towards their lower ends, and some of them 

 towards their free ends also. The nuclei are only seen in the profile view of the cells and 

 are variable in position, being usually in the widest part of the cells, sometimes near the 

 free end but sometimes deep down The surface view of these cells has the appearance 

 of a very fine mosaic. 



The central brown ana constituting the bottom of the groove is composed of extremely 

 long columnar epithelium apparently not ciliated. The cell elements are here rather 

 difficult to distinguish. They are very greatly elongated, and are columnar or fusiform in 



(ZOOL. CIIALL. EXP. — PAST XYIL— 1882.) II 1-t 



