ANATOMY OF PELECYPODA BURNE. 21:: 



The lips and palps resemble those of Li**ur<;i and .\<l<inn/r<',i. 



and, as in those- two genera, tin- out or palp is I'm- si of its oxtrnt continent witli the 



mantle, its extreme edge only projecting frcelv (see l.ixxiin-d. Fig. 5 n. ./<///.). It is 

 also worthy of record that in Plnl<>l>ri/<<. as in the two above-mentioned species, there 

 is a well-developed labial c;i;eiim, apparently of a glandular nature, opening to the 

 exterior in the angle between the inner lip and the bodv-wall (Fig. |:i. /.<.). 



It is to be noticed, too, that in Philobrya, as in f.ixmn-t/ and Ai/in-n/ir/-<i (the onlv 

 three genera in which, so far as observed, labial e;vea occur), the epithelium of the inner 

 side of the gill-axis is increased in depth and forms a prominent sub-branchial ridge 

 (Fig. 14, ep.r.), similar, both in extent, form, and structure, to that alreadv desci-ibed 

 above in the account of L/xxitrct'i (Fig. 7, cp.r.). 



Sense-Organs. At the extreme anterior end of the inner demibranch, at the point 

 where it is attached to the inner palp, is a " cephalic " eve, well formed and of some 

 considerable size. As is usually the case with this vestigial organ, it has the form of 

 an open cup of deeply -pigmeuted cells, filled in by a mass of homogeneous transparent 

 substance (Fig. 15). 



Hi'nrt. The ventricle of the heart is perforated by the intestine; and behind the 

 ventricle, as described by Pelseneer for /'. SVJblcevis, the two auricles communicate with 

 one another by a wide passage (Fig. 13, KH.I:). The connection between the auricles 

 and ventricle is quite normal, there being no indication of the double communication 

 on the left side found by Pelseueer (probably as a variation) in his specimen of 

 /'. xit/ifirrix. 



Renal Organs. The renal organs and genital duct are closely similar in structure 

 and relations to those of Lissarca and Adacnarca. 



Gills. The gills in their disposition and in the details of their structure resemble 

 those of Adacnarca, except that in Philobrya the reflexion of both demibranchs is 

 more extensive, and the reflected angle of each filament is strengthened bv a short 

 inter-laminar web. The cavity of each filament is divided longitudinally bv a septum. 



In view of the several particulars just mentioned (palps, labial cseca. sub-branchial 

 epithelial ridge, gill-structure), in which Philobrya limoides shows a marked similarity 

 of structure to AiJni-niin-ii, it is of some interest to find that there is also a close 

 likeness between these two genera in the disposition of the mantle-cavity in relation 

 to the anterior part of the gills. 



It was pointed out above that in Adacnarca the mantle-cavity extends upwards 

 for some distance between the separated anterior attachments of the two demibranchs, 

 and that further back this extension is cut off from the general mantle-cavit v bv the 

 transference of the attachment of the outer demibranch from the mantle to the bod\- 

 wall, and runs back for some distance as a blind pocket between the kidney and the 

 surface of the body (Figs. DA, B, ', nt.<:). In Phildbrya the relation of the mantle- 

 cavity to the gill-attachment is precisely the same. There is a similar upward 

 extension of the mantle-cavity between the separated anterior attachments of the 



2 P 2 



