ii. 
the Nautilus pompilius to the last. This is the only remnant of the 
order which once inhabited every sea of the world. The Pelecy- 
poda and the Brachiopoda resemble each other in both having a 
bivalve shell ; but differ in the attachment of the animal to the 
shell. In the former it is lateral, and marked by one or two 
muscular scars on the inside face of each valve. In the latter it 
is vertical or dorsal. The Pelecypoda have no distinct head ; 
the absence of a Jaw is compensated for by a series of vibratory 
ciliz, which fringe the gills, creating currents, propelling the 
food in the direction of the mouth, into which it is guided 
by palpi on each side. As the Order is exclusively aquatic, and 
the animals branchiferous, their uniformity of life determines 
their anatomical characters, which are more marked than 
with the Gastropods. The Pelecypoda usually rest on the 
longer axis of the shell, and with the aid of the foot can burrow 
vertically down into the sand, or mud below. Some choose for 
their homes the tough tegument of an Ascidian. The Pholadide 
can perforate solid rock by means of the rasping file-like ridges of 
the shell, which they have the power of revolving, and of hollowing 
out a highly-polished cist. A colony of several will occupy the same 
block of stone. Gastrochzena, Saxicava, and other borers whose 
shells are smooth, have also the power of excavating in the solid 
rock without any visible appliance, which is possibly done by 
chemical action. The Mytilus and Modiola spin a byssus, by 
which they are able to attach themselves to some object. These 
frequent the shoreline, and thus anchored are not exposed to be 
washed away by the incoming sea at low-tides. 
The Gastropods receive their name from the situation of the 
muscular dise or foot, upon which the animal creeps. The aquatic 
species are usually furnished with an operculum attached to the 
posterior surface of the body, completely shutting it in when with- 
drawn within the shell. Fomatias (Cyclostoma) elegans and Acicula 
lineata are the only British land-shells with this appendage, and are 
probably the survivals of aquatic ancestors. The shell of Helix 
aperta is so disproportionate to the size of the animal that it can 
