so-called Eyes o/Tridacna. 441 



transparent cells those of the outer layer appear very opaque, 

 owing- to their coarsely granular protoplasm ; they are some- 

 what smaller (10-15 /*) than the transparent cells, show no 

 recognizable membrane, and are polygonally pressed against 

 one another. Their round nucleus, averaging 3-5 //. in 

 diameter, is not placed excentrically, but more in the centre. 



It is exceedingly remarkable and ought to be particularly 

 noted that I have never seen a nerve passing to a flask-shaped 

 organ. 



With the anterior neck-like portion the organs frequently 

 reach immediately under the epithelium, but just as often the 

 thin layer of connective tissue, which forms the boundary 

 between the tissue of the mantle and the epithelium, inter- 

 venes between them. The epithelium is the moderately 

 thick, one-layered, vibratile cylinder-epithelium, with basal 

 nuclei and thin, strongly refractive cuticle, which is suffi- 

 ciently well known in the mantle of the Lamellibranchs. On 

 the outer surface of the mantle (the shell-side) a great num- 

 ber of elongate, flask-shaped, unicellular mucus-glands open 

 among the epithelium ; these, when stained with alum- 

 carmine, show very distinctly the framework-substance 

 recently described by List * and myself in these elements. 



The flask-shaped organs often project outwards more or less 

 strongly, so as to push out the epithelium covering them in a 

 conical form. This peculiarity is generally much better deve- 

 loped in those organs which we tind upon the smaller (and 

 probably younger) warts than in those which belong to older 

 structures. With regard to their epithelial coat also a dis- 

 tinction may be set up. Throughout this is somewhat 

 thinned over the flask-shaped organs, but in the younger much 

 more than in the older ones. \Vliile in the latter the differ- 

 ence from the normal epithelium is but small and often scarcely 

 perceptible, the epithelial covering over the younger struc- 

 tures is olten reduced to a pavement-epithelium scarcely 

 visible in profile. 



We have just spoken of older and younger warts, and this 

 gives the opportunity of going somewhat more into detail 

 upon the relations of the two structures, which in description 

 we have in the first place treated separately. If we examine 

 the minute structure of one of the undeveloped structures 

 near the margin of the mantle, which appear to the naked 

 eye rather as scar-like indrawn depressions, it is soon seen 

 that no principal diflierences from the larger warts exist in 



* J. H. Liyt, " Zur Kenntniss der Drijsen im Fusse von Tethys Jim- 

 briata, L.," Zeilsclir. f. wir^s. Zool. Bd. slv. p. 281 ; and J. Brock, ibid. 

 x\i\. p. 3^3. 



