MUSCLES — INVOLUNTARY AND VOLUNTARY. 
841 
As we ascend the scale in the animal king<loin or arrive at a later 
stage in the development of the mollusca, true locomotor and other 
muscles become more distinctly differentiated and it hecomes due to 
their alternate contraction and extension that movements are effected. 
Muscles in most animals are what are popularly known as flesh, and 
are constituted by bundles of fibres collectively enclosed by a strong 
membrane, each constituent fibre being surrounded by a thin sheath 
of connective tissue and arranged with its length parallel with the 
direction or pull of the muscle, and also connected with a nerve cell, 
whose stimulation causes the contraction of the filament and draws 
together tlie parts to which the opposite ends of the muscle are affixed. 
Muscles may be Voluntary or Involuntary, according as they are 
or are not under the control of the animal, although it is probable 
that many apparently voluntary muscular movements are merely 
reflex actions in response to external stimuli. 
Involuntary or Unstriated Muscles are predominant in the 
molluscan organism, but are more es- 
pecially characteristic of the splanchnic 
muscular layer, and are composed of 
greatly elongated, smooth and unstriped 
muscle-fibres, which generally contract 
slowly and rhythmically, as exemplified in the regular and periodic 
movements of many of the internal organs. 
The Voluntary or Pseudo-stri.ate Muscles in the mollusca are 
chiefly retractors, but also embrace the integumental muscles, tlie 
protractors, the levators and the extensors, but the protractor and 
other muscles are in their action usually subsidiary to concentrated 
blood pressure in causing the 
protrusion of the extensible or- 
gans of the body. 
In the mollusca, the Voluntary 
muscles show transverse granu- 
lation or are entwined by spiral fibrillm which give to the muscular 
filaments a striated aspect. This incipient transverse striation is 
especially characteristic of molluscan voluntary muscle, being pre- 
sent in the buccal mass of many Gastropods, and in the adductors of 
the Pelecypoda. It is the biological expression and mechanical 
result of their increased function and of the numerous waves of 
contraction of the muscle substance, as the striation is incomplete 
Fig. 633. — Unstriated muscle fibre 
from foot of Neritina Jiuviatilis 
(L.), highly magnified (after Boll). 
Fig. 634. — Pseudo-striate muscle fibre from 
buccal bulb of N'l’f'itina /Juviatilis highly 
magnified (after Boll). 
