lO HoYLE, PterygioteutJiis margaritifera. 



there is no definite line of demarcation between them. 

 The anterior portion of this cap is rather difficult to make 

 out satisfactorily owing to the defective state of the speci- 

 mens. In only a i&v^ instances is the surface intact, and 

 here it is seen to have the following arrangement. 

 Over the outermost scales is a layer of fibrous tissue, 

 similar to that which lies between them and this again 

 is bounded by a layer of thin deeply stained scales, 

 to all appearance of the same nature as the other. 

 Immediately outside these is a very thin membrane 

 which in its turn is covered by a fibrous tissue, 

 becoming more dense outwards and, as it were, condensing 

 into a definite membrane. Presumably in the living 

 condition this is covered by an epithelium, but no trace of 

 this remains in any of the preparations. 



At one side of the organ the external membrane 

 separates from the underlying delicate membrane so that 

 a considerable space, triangular in section, remains between 

 them {Fig. 3, j.). This is occupied by a delicate fibrous 

 tissue with a good many lacunae in it. One of these 

 lacunae is much larger than the others (/), and in 

 its neighbourhood are several blood vessels, though it 

 does not seem to have been a vascular sinus itself Over 

 this thickened part of the anterior cap and lying just 

 within the outermost fibrous layer are a number of deeply 

 stained round bodies {Fig. 3,/), which are either nuclei or 

 the sections of fibres. 



II. TJie Siphonal Organs (Fig. i, s.s.) are two in 

 number, situated just below the hinder margin of the 

 siphon and visible peeping out from beneath it when the 

 mantle is divided in the middle line and laid open. Like 

 the branchial organs, these are in the natural state covered 

 by the mantle, and are onl\- effective by reason of its 

 transparency in the living animal. They are spheroidal 



