MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 473 



capped by a tissue which stains brightly, and consists mainly of ciliated supporting cells, but 

 is not sharply separated from the unmodified mesenterial epithelium. The mesog-lo?a is not 

 swollen or ))ifurcated, and the eiidoderm immediately behind never becomes lobed. Such 

 incipient filaments ocour on the secondary mesenteries of Orbicella and Cladocora (fig. 57a), and a 

 somewhat further stage is represented l)y Saleinisfr^n (fig. 85). The figures should be compared 

 with the early stages in the development of the filament met with in larxie (PI. XV), and also 

 in l^or'deH (fig. 38). There is the closest resemblance between the two phases, leading to the 

 conchision that phylogenetically they represent similai' structures, whether continuous with the 

 stomodanil ectoderm or remaining free from it. 



GL.\NDITLAR MODIFICATIONS. 



The filaments of many species of corals undergo a peculiar histological modification, the 

 organs within restricted limits becoming almost wholly glandular. In the fresh tissues, the alter- 

 ation is indicated by the part being of a golden yellow color, instead of the usual dull white; while 

 in preserved material the same parts are much darker than the rest of the filament, above and 

 below. A transvei'se section through one of these modified regions, taken from OrhiccJlu aim uJuris. 

 is represented on PI. IX, fig. 69. The filament has become enlarged in diameter, and its cellular 

 constituents are remarkably uniform in character. The latter are long, clearly defined, columnar 

 cells, radiating in a fan-like manner from the expanded mesoghcal base. Each cell is filled with a 

 finely granular substance, and on staining a nucleus is rendered visible. The free margin presents 

 no indication of ciliation, but, in places, globules of some liquid appear in the act of oozing out, 

 while the organ is enveloped in some secretion, evidenth" issuing as the polyp was preserved. 

 The secretion is of a faint yellowish color, slightly difl'erent in refraction from the Canada 

 balsam in which the sections are mounted. The same filament, as it appears in a partly 

 tangential section, is represented in fig. 70. The cells in the middle are cut transversel}', while 

 peripherally they tire seen more lengthways. The well-defined polygonal outline of each cell in 

 transverse section is very characteristic. 



After maceration, the preparations (fig. 71) reveal that the filament comprises only two 

 kinds of cells: (a) long, columnar gland cells, of the same diameter throughout, and charged 

 with granular matter; and {h) equally long, narrow supporting cells. Nematocysts are altogether 

 wanting. The modification extends over a ver}' restricted vertical range, for on following the 

 sections of the filament, both upward and downward, the normal, more complex structure soon 

 appears. 



The actual presence in some cases of a secretion surrounding the filament, and tlie character 

 of the cells themselves, leads to the conviction that the structure represents a purely glandular 

 organ. Such a histological specialization is very exceptional among Zoantharian tissues. One 

 of the functions of the ordinary mesenterial filament is deemed to be the production of a 

 digestive secretion, and it is manifest that in these special filamental regions an increase in size 

 and number of the secretory cells has taken place, to the exclusion of nematocvst and other cells, 

 with the exception of the ever present supporting cells. 



A comparison with the section through the unmodified region of the same filament, repre- 

 .sented in fig. T^, at once suggests the manner in which the alteration has taken place. The 

 portion of the mesentery included in the figure is at first very narrow, but just behind the 

 filament its epithelium and mesogkea become swollen, and as the latter enters the filament it is 

 flattened, terminating in a branch to each side. The boundary between the filament and the 

 swollen mesenterial endoderm is clearly defined. The comparkson of fig. 61t and fig. 72 renders 

 it evident that in the former the whole of the filament has taken on the glandular character, 

 while the unmodified basal area is the swollen mesenterial epithelium, now. however, so closely 

 apposed to the filament, as to be distinguished only histologicall}-. 



Intermediate stages in the production of the glandular organ from the normal filament are 

 afforded b}' the filaments of 3[;r<iii</riiiti (PI. XXI). A transverse section of one of these is 

 represented in tig. 144; the right side of the filament presents the usual histological details, while 



