252 A MONOGRAPH OF THE AUSTRALIAN APHRODITEA, 



row. Each elytron is attached to a peduncle or scale-tubercle, 

 the surface of attachment, of circular or oval form, being situated 

 about the centre of the elytron, and the attachment being effected 

 through the medium of a series of muscular fibres, by whose 

 contractions the various movements of the scale are brought 

 about. The elytra are usually attached to every alternate 

 segment — the intermediate segments having as their equivalent 

 the cylindrical dorsal cirri. The dorsal cirri and the elytra may 

 be said to be essentially identical in structure and mode of 

 development. Each consists of an evagination of the integument 

 containing a nerve, the evagination in the one case taking a 

 cylindrical form, while in the other it becomes compressed and 

 scale-liko. Such a cutaneous fold ought to contain representatives 

 of the cuticular, the subcuticular and tlio muscular layers of the 

 integument, and such we find to be actually the case. Each scale 

 contains three principal elements: — (1) an investing cuticle, (2) 

 a double layer of cells or cell-equivnlents, and (3) a fibrous layer. 



(1). The cuticle varies considerably in its degree of develop- 

 ment in difierent genera and species. Sometimes, as in Aplirodita 

 and Hermione, it is exceedingly delicate and developes no 

 appendages ; sometimes as in Iphione and in many species of 

 Lepidonotus and allied genera its upper layer attains a consider- 

 able thickness and density, and may be variously sculptured on 

 the upper surface ; where appendages, such as papillae or fimbria), 

 are present, it forms an investment for them when it does not 

 constitute their entire substance. 



(2). The cell-elements representing the subcuticular layer 

 of the integument take the form of a layer of polygonal 

 squames lying immediately under the cuticle. This layer ia 

 sometimes transparent — the outlines of the cells being only 

 discernible with difficulty ; at other times the cells are charged 

 with pigment granules, a lighter space in the centre being 

 apparently the expression of a nucleus. This double layer of 



