72 
J. D. MACDONALD ON METAMORPHOSIS OF GASTEROPODA. 
shell. Ample provision is made for this contingency hy a dense epithelial pavement, which 
lines the cartilaginous shell, and which may possibly be homologous with that by which the 
test is evolved in the Tunicata. As the chief agent in the development of the outer shell, it 
may be termed the outer mantle ; and there is much reason to believe that it is persistent 
the adult Brownia and in many other 
It is probably within an outer mantle 
homologous with this, that the first rudiments of the shell are formed in the JHelicida, 
and the outer thickened rim of the mantle in the adult animals may be the remnant of 
such a structure. Its coexistence with the true epipodia in Aplysia and other instances 
precludes the notion that it may be altogether composed of these processes which have 
coalesced over the back. These facts and considerations afford a simple explanation of 
the development of internal shells in general, not even excluding those of the Cephalopoda, 
into which the homologues of the retractor muscles of the Gasteropoda are fixed, although 
the shells themselves in the two Orders are not generally admitted to be homologous, even 
against the light which the scutellum of the slug is calculated to throw on the subject. 
I must not omit to notice the capture of a minute Pleurobranchus in company with the 
pelag 
forms, off the Cato Heef. It was entirely naked, with broad ciliated head-lobes 
or festooned folds, apparently continuous with the epipodia. The large and beautiful 
labial plates exceeding the dimensions of the double lingual pavement of uncinate teeth, 
and the auditory sacs (in this stage containing single but minute otoliths) lying wide 
apart from the eye specks, at once distinguished it from Doris and other allied genera. 
Besides the numerous groups yet to be determined as fulfilling the conditions set forth 
in this paper, I believe that I have already identified members of no less than six or seven 
natural families, which may be distinguished by the simple characters given in the fol- 
lowing Table. 
PELAGIC GASTEEOPODA (so called). 
With strap-like dentition and otoliths in the ear-sacs. 
f Outer lip en- 
Shell 
& 
Operculum . 
tire. 
Outer lip 
with claw- 
^ like lobes. 
Operculum with a strong spine 
in front Strombid^e 
< 
Operculum ^ 
simple. 
Nucleus minute, 
not continuous 
with the sub- 
sequent lines of 
growth Ranellid^e 
Ciliated 
arms 6. 
Nucleus large, 
continuous with 
the lines of 
^ growth Naticidje 
> Ciliated 
arms 4. 
f Cancellated . . * ? 
Calyptr^id^e 
& 
Cypr^eidje. 
Double-keeled MuricidjE. 
Smooth 
BlJCClNIDiE, 
Ciliated 
arms 4 
^ 
Dentition 
f septiserial 
^ 
\ Animal 
Dentition 
triserial 
To these may be added Brownia, and those species which are included in a cartilaginous case. 
