STRUCTUEE, ETC., OF CERATA OP NUDIBRANOHS. 45 



part, while the second pair of cerata are seen one on each 

 side. This shows well the laminated structure of the branchia 

 and the bundle of muscle-fibres branching through its interior. 

 A few spaces are visible in the branchia, but they are com- 

 paratively small, and it is only under a higher magnification 

 that the numerous lacunae lying in the mesoderm close under 

 the ectoderm, and containing blood-corpuscles (fig. 8), become 

 visible. This is part of a vertical section ; while fig. 9 shows 

 part of a transverse section similarly magnified. These two 

 figures show the deep infoldings of the surface of the branchia, 

 and the former (fig. 8) exhibits well the change in the character 

 of the ectoderm cells from place to place. The general arrange- 

 ment and structure is the same as in the section of the 

 branchia of Doris (fig. 2), and is very different from the struc- 

 ture of the cerata when similarly sectionised and magnified 

 (see fig. 10). So that, although in sections such as are repre- 

 sented in figs. 7 and 11 the cerata and the branchiae sometimes 

 overlap and become displaced, small pieces of the one are 

 always distinguishable by their structure from those of the 

 other. The cerata (see figs. 6, 7, 10, and 11) have the ecto- 

 derm very thick, and the infolds are not nearly so deep or so 

 close as those of the branchiae. A layer of longitudinal muscle- 

 fibres (fig. 10, m) lies under the ectoderm in the cerata, and 

 there are only a few small lacunae in the mesoderm. 



Fig. 11 represents a section further back, in which parts of 

 all three branchiae and of two pairs of cerata are seen. The 

 lateral branchiae have their inner surfaces much more deeply 

 folded than their outer surfaces, and this is especially the case 

 near their bases. This is shown in fig. 12, a vertical sec- 

 tion of the base of one of the lateral branchiae, where the left 

 side shows the outer surface next to the cerata, while the right 

 side is the inner surface nearest to the middle line of the body. 

 Some of the deepest infolds of the ectoderm are seen to end in 

 little crypts where the ectoderm cells become suddenly large 

 and are arranged in a radiating manner around the end of the 

 infold so as to form a spherical clump (fig. 12, gl). These 

 are probably glandular. 



