240 Mr. W. A. Haswell on the 
It has been generally assumed by Quatrefages*, among 
others, that the scale is a flattened sac, between the two walls 
of which is a narrow cavity communicating through the-scale- 
tubercle with the cavity of the body. Hvidence in favour of 
this supposition is afforded by the fact that, in the case of 
certain species, specimens have been observed with all the 
scales distended and globular, as if blown up by the pressure 
of fluid from within. J have never seen this phenomenon, 
which seems to be very rare; it is probably due to a forcible 
contraction, similar to that which causes the throwing-off of 
the scales, forcing the perivisceral fluid through the scale- 
tubercle into the space between the two layers, and causing a 
rupture of the intermediate tissue. 
That this is the true explanation of the phenomenon in 
question will be evident if we examine the structure of the 
scales in Aphroditat. Here we find that the two membranes 
of which the scales are composed are firmly united together by 
fibrous tissue, and require some little force to separate them. 
This fibrous layer is visible in the undissected scale by focus- 
ing through the external membrane, and is seen still more 
distinctly when the two membranes are separated, when the 
torn fibres will be seen curled up on the inner surface. This 
central tissue consists of exceedingly fine fibres, which are 
sometimes arranged in definite interlacing bundles, while in 
other instances they cross one another irregularly in all direc- 
tions. Morphologically this layer may be taken to represent 
the muscular layer of the integument. 
In his account of the structure of the nervous system in 
Aphrodita aculeata t, M. de Quatrefages makes no mention 
of the existence of nerves in the elytra. Ehlers§ seems to 
have been the first to observe their presence in the elytra of 
Polynoé. In Polynoé pellucida he found a nerve entering the 
scale through the scale-tubercle, and branching throughout 
the scale. A similar arrangement is very well seen in the 
scales of Aphrodita and some species of Lepidonotus and 
Polynoé after they have been strongly stained with hema- 
toxylin or cochineal. ‘The nerve divides near the point of 
entrance into numerous branches, which radiate towards the 
periphery and become divided again and again, giving off 
numerous minute twigs. The termination of many of the 
ultimate twigs in relation to the processes on the surface of the 
* Histoire Naturelle de Annélés, t. i. 
+ A. australis, Baird, the Australian sea-mouse. 
{ “Sur le systéme nerveux des Annelides,” Ann. des Sci. Nat. 3° 
série, t. xiv. p. 362. 
§ Die Borstenwiirmer, p. 110. 
