CEPHALOPOD ANATOMY

ADVANCED INVERTEBRATES

COMPLEX, HIGHLY DEVELOPED NERVOUS AND sensory systems are typical features of living cephalopods. Especially noteworthy are the image-forming eyes and the complex brain, which has evolved from the nerve ring around the esophagus of other molluscs. The ability of most cephalopods to change their appearance rapidly because of specialized pigment organs and reflector cells in the skin that are under direct nervous control makes observation of living animals fascinating. Other noteworthy peculiarities, such as movement by muscles squeezing other muscles, are also characteristic of cephalopods.

EXTERNAL STRUCTURE

Three regions can easily be discerned on a cephalopod. From anterior to posterior, these regions are:

1) the arms and tentacles surrounding the mouth

2) the head, on which the eyes are prominent

3) the sac-like mantle, or body, to which fins may be attached. This overall structure is less distinct in nautilids, which have external shells into which they can withdraw, but is still recognizable.

THE CROWN OF ARMS

Cephalopods other than nautilids have either eight or ten arms surrounding the mouth. On those with ten, two arms are modified into either tentacles (decapods) or filaments (vampires). The arrangement of the arm crown is different in nautilids from that in all other living cephalopods. Nautilids have approximately 90 arms arranged in two rings around the mouth. Each arm is retractable within an outer sheath. The oral surface of each arm is covered by adhesive transverse ridges, rather than by the suckers typical of other cephalopods, which have either eight or ten arms surrounding the mouth. Hence, all living cephalopods other than nautilids have eight arms and some additionally have two tentacles or two filaments.

TENTACLES

The most obvious difference between arms and tentacles on a decapod is in the arrangement of armature (suckers, some of which may be modified into hooks). The arms of most cephalopods have one, two, or four longitudinal series of suckers or hooks extending along the entire oral surface. Tentacles tend to be longer than arms and to have the armature concentrated toward the far end, which is often expanded to form a tentacular club. The filaments of vampires are very long and thin with no armature, and can be retracted into pockets.

Although both decapods and vampires have eight arms and two modified appendages, those modifications are not of the same two appendages. In decapods, the tentacles are ventrolateral, whereas in vampires, the filaments are dorsolateral. Although many people think that the two arms that have been lost in the octopods, out of the general ancestral pattern of ten arms, are the tentacles as found in decapods, it is more likely that it was the dorsolateral arms that are modified in vampires that were lost in octopods.

SUCKERS

Suckers are of exceptional use to cephalopods — the arms and tentacles essentially have the task of delivering the suckers to a destination to conduct feeding, tasting, attachment, or movement. Basic differences exist in the structure of suckers among the major groups. Octopod suckers are radially symmetrical with cylindrical bases that are either broader than the suction cup or only slightly constricted. Decapod suckers are bilaterally symmetrical and their bases are narrow stalks. Therefore, an octopod sucker may be described as looking like a volcano whereas a decapod sucker looks a little like a ball on a stick.

The surface around the inner opening of an octopod sucker has a thin cuticular lining, while the opening of a decapod sucker is characterized by a rigid chitin-like ring that digs into captured prey upon suction and resists the shear forces caused by a wriggling fish or shrimp. These rings may be smooth, notched (referred to as "blunt teeth"), or serrated ("sharp teeth"). The tooth-like structures are of unequal sizes around the opening of the sucker. The result can be elongated sharp teeth in one area of the sucker ring and either blunt teeth or a smooth region on the opposite area. In several squid families, the central elongated tooth in some suckers becomes hyper-developed, forming a hook reminiscent of a cat's claw.

Conical muscular structures, like little fingers, are associated with the bases of the suckers in cirrate octopods, vampires, and most decapods. In cirrates and vampires, both of which have suckers in a single series, these structures are called cirri (singular = cirrus) and are found in pairs alternating with the suckers along part, or all, of the arm. The similar structures on decapods are called trabeculae (singular = trabecula) and extend from the outer bases of the outer series of suckers on the arm or tentacle. Cirri and trabeculae very likely evolved from the same ancestral structure.

In many decapods some suckers in the proximal part of the tentacular clubs develop as simple knobs, alternating with normal suckers. When the two clubs are held together, the knobs on one tentacle align with the matching sucker on the other tentacle and each sucker in that part of the club can attach to a knob. Hence, the tentacular clubs can be attached together at their bases by this overall structure known as a tentacular locking apparatus.

WEBS

In addition to the suckers, the arms have a variety of external structures. Membranes often border the oral surfaces of the arms. Because their function appears to be protection of the suckers, these structures are often described as protective membranes. When these membranes are continuous between adjacent arms, they are called webs. The webs are quite deep in some species, essentially connecting the entire arm to its neighbor. Webs in some species can be very thick and fleshy.

On many decapods, the trabeculae are embedded in the protective membranes, in which case they may be referred to as trabecular membranes. Decapods also have a ridge on the surface of the arms and tentacular clubs away from the suckers. These ridges, known as keels, provide hydrodynamic lift to the head/arm end of the animal when swimming tail-first. The keels on the ventral arms of some squids (such as chiroteuthids and mastigoteuthids) are shifted in position to form a groove in which the thin tentacles are held.

THE MOUTH IN THE MIDDLE

Decapods have a structure around the mouth somewhat similar to an inner ring of arms. A membrane around the mouth, the buccal membrane, is supported by six to eight muscular pillars called lappets. The lappets are like little arms; in some taxa (including some cuttlefishes and inshore squid species, and the family Bathyteuthidae) the lappets have tiny suckers like miniature arm suckers. All functions of the buccal membrane are not known, but in some species sperm packages are implanted on the buccal membrane of the female during mating.

In all cephalopods...