The anatomy of pleurotomarta bbyrichii. 225 



met within the more lowly Lam ellibranchs, viz. the Proto- 

 branchia (Mitsukuri ^). Consequently one might naturally 

 expect to find a similar resemblance in the finer structure of 

 these two organs. In this, however, one is doomed to disap- 

 pointment, for although at first sight there appears to be a 

 great resemblance (cf. Mitsukuri, op. cit., pi. xxxiv, figs. 6 

 and 8, and my figs. 17 and 18), yet when we examine this 

 more carefully we find that instead of a resemblance, there is 

 in reality a very marked difference. Thus in Nucula the 

 supporting rods lie along the ventral border of the gill-plates 

 and meet along the ventral edge of the gill-septum, whereas 

 in PI euro torn aria, as we have seen, the gill- skeleton is 

 situated along the outer or dorso-lateral margin of the plates, 

 and the connections between the adjacent rods take place at 

 the dorsal attachment of the plates to the septum. Simi- 

 larly, the modified ciliated epithelium, which is closely related 

 to these rods, is dorsolateral in the Gastropod, and ventral in 

 the Lamellibranch. We thus see that there is a very strik- 

 ing and fundamental difference in the relation of the gill- 

 skeleton in the two groups, and one which must tend to throw 

 back the common ancestor of the two to a still eai'lier period 

 than that generally assigned to it. 



The dorso-lateral position of the supporting rods is, how- 

 ever, found in another great Molluscan order, the Cephalopoda. 

 Thus in Sepia Burne ^ has described cartilaginous rods, one to 

 each gill-plate, strengthening the supporting lamella, in a 

 position corresponding to the outer and dorsal margin of the 

 gill-plate of Pleurotomaria. This is an interesting point, 

 for, as we shall see later, the Diotocardia appear to approach 

 the Cephalopoda further in the relation of the spiral stomach- 

 caecum. The skeletal difference between the gills of the 

 Diotocardia and the Protobranchia is, however, far more 

 surprising than the resemblance of the former to the Cepha- 

 lopoda, for, in addition to the general form and relation of 



' "On the Structure and Significance of some Aberrant Forms ol' Lanielli- 

 brancliiate Gills," ' Quart. Journ. Micr. Sci.,' vol. xxi, 1881. 

 ' 'Proc. Mai. Soc.,' vol. iii, p. 53. 



VOL. 44, PART 2. — NEW SERIES, P 



