280 GILBERT C. BOURNE. 



suprabranchial or tlie pallial cavities, and some of them, such 

 as those marked z in figs. 11 and 13, end in veritable culs- 

 de-sac. The entrances to these cu Is -de-sac is guarded 

 by two ridges, whose thickened borders are covered by a 

 columnar epithelium furnished with stiff cilia. A transverse 

 section of one of these ridges is shown in fig. 8. There can 

 be little doubt that when the thickened borders of these 

 ridges are in apposition the cilia interlock, and thus close 

 the entrance to the cul-de-sac, which then serves as a 

 reservoir for water. The largest of these reservoirs are 

 situated between the hinder part of the right labial groove 

 and the visceral mass (fig. 11, %.), between the right kidney 

 and the right mantle lobe, below the level of the heart (text- 

 figure 2, F, 2.), and between the hinder part of the intestine 

 and the right mantle lobe (fig. 13, z.). It is evident, then, 

 that Enigma is well provided against the danger of 

 desiccation when exposed for days together to the rays of 

 the sun. 



The most remarkable of the pallial organs are the pigment- 

 spots or eyes of the left pallial lobe, whose position has been 

 described on p. 258. The structure of these organs is alto- 

 gether peculiar in that two epidermic layers, namely, those 

 of the outer and inner face of the mantle, share in their 

 formation. As is shown in fig. 20, each eye comprises a 

 cornea, a lens, a vitreous body, and a layer of deoply- 

 pigmented cells forming a rudimentary retina. The cornea 

 is formed by a local modification of the external epithelium 

 of the mantle, which, elsewhere formed of columnar cells 

 with an admixture of glandular cells, here becomes flatter ; 

 its component cells are transparent and vacuolated, and the 

 cell outlines are scarcely distinguishable, but the nuclei are 

 distinct and oval, with scattered chromatin granules. The 

 lens (fig. 21) is a biconvex lenticular mass lying below the 

 cornea, but separated from it by a thiu sheet of tissue 

 continuous with the vitreous body. The lens, in spirit 

 specimens, is of an opaque Avhite colour, and no definite 

 structure can be discerned in it. It is a vacuolated mass of 



