Longitudinal muscle fibers depend on calcium influx into the cell for excitation-contraction coupling, while circular muscle fibers rely on intracellular calcium release.
12.
Because the volume of the tentacle remains constant, contracting the circular muscles decreases the radius and permits the rapid increase in length.
13.
Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large.
14.
The lateral nerve cords are outside the circular muscle, as in palaeonemerteans, but separated from the epidermis by the usually well-developed outer longitudinal muscle.
15.
Segmentation involves contractions of the circular muscles in the digestive tract, while peristalsis involves rhythmic contractions of the longitudinal muscles in the GI tract.
16.
Unlike peristalsis, which involves the contraction and relaxation of muscles in one direction, segmentation occurs simultaneously in both directions as the circular muscles alternatively contract.
17.
The lens of the eye is suspended in the eyeball right behind the iris, connected by a halo of fibers to a ring of circular muscle.
18.
Peristalsis, involuntary movements of the longitudinal and circular muscles, primarily in the digestive tract but occasionally in other hollow tubes of the body, that occur in progressive wavelike contractions.
19.
A less common example would be a circular muscle such as the orbicularis oris, in which the fibers are longitudinally arranged, but create a circle from origin to insertion.
20.
As with the Spirulidae and Idiosepiidae, the Oegposina lack suckers on the buccal supports, and like the Bathyteuthoidea, Idiosepiidae, and Spirulidae, they have no circular muscle on the suckers.
