ACEPHALOUS MOLLUSCA. 
321 
The Acephalous or Headless Molluscs are so called from the Greek 
“> privative, aud KefiaX?), head. They have no head ; the body is 
Grounded by the folds of the skin ; the shell consists of two valves. 
^ Uc h is a summary description of all the Acephalous Molluscs. They 
are sometimes naked, and sometimes enclosed in a shell or test, 
^hence they are known as Testaceous Molluscs. They are called 
Waives, because their shell consists of two halves, or valves united by 
a tinge. They are sheltered in this double carapace as a book is in 
l fo cover. 
Although they have no head, they can feed themselves ; they have 
deling of' some sort, and they reproduce their kind. They have 
headships and enmities, perhaps even passions ; probably these are not 
Ver y lively, for most of' them scarcely ever change their place, even to 
!lUj Te the least movement. Many of them remain fixed to the rock on 
^tick they were hatched, and tumultuous sensations are not quite 
c °>npatible with immobility. 
The bivalves are found in every sea. The shell of the bivalve is 
' f ‘ v °id, globulous, trigonal, heart-shaped, elongated like a pea-pod, or 
fol like the leaves of a tree, having an opening down the ventral side. 
A 1 some one leaf is flat, the other round and swelling in the centre, 
/foe shell is thus an outer envelope, consisting of two pieces, more or 
corresponding to each other in size aud shape (of which the oyster 
' s a n example), formed of carbonate of lime deposited in membranous 
Ce ^ s in its outer layers, the inner layers being composed of thin coat- 
lu § s of lbne deposited in the outer surface of the tissue, called the 
tie-leaves. The valves are united to the animal by the insertion 
c ertain muscles, and by the horny epidermis of the mantle, which 
re tehes over the edge of the valves. The hinge aud ligament which 
fo® the two valves consist of a dense elastic substance, somewhat 
^embling india-rubber ; the hinge is formed of teeth, and cavities into 
^ lc T the teeth fit. The ligament acts in opposition to certain con- 
a ctil e muscles within, which draw them together, and is placed either 
or without the hinge, or partly both. On separating the valves, 
'foli, 
, two leaves of the mantle present themselves. These are thin 
Ca te leaves, furnished at the margin with sensitive tentacles and 
QfL . _ 
. er organs of sense, and with glands sometimes highly coloured. 
e use of these organs is thus described by Mr. Eymer Jones : — ■ 
the animal is engaged in increasing the dimensions of its 
The 
