Cephalopoda

Suckers

stalked, cup-shaped, muscular; cover the inner surface of each arm

Beak

Parrotlike mouth formed by large dorsal and ventral jaws

Ink sac complex

absent in nautiloids; consists of ink gland, ink sac, and ink sac duct

Systemic heart

dedicated to pumping blood to the body; heavily muscularized organ consisting of a central ventricle and a pair of atria

Branchial hearts

dedicated to providing the gills with pressurized blood; contractions force blood through the afferent branchial vessels and into the gills

Epidermal lines

analogous to lateral line system; consists of hair cells sensitive to weak water movement and pressure waves

Chromatophores

multicellular organs consisting of a central pigment cell and numerous small radial muscles under nervous control

Iridiocytes

individual cells located in the dermis, iris, and ink sac; elliptical cells whose extensively folded plasma membrane is a diffraction grating that refracts light of specific wavelengths although no pigments are present

Photophores

may be bacterial, in which the light is produced by symbiotic bacteria, or intrinsic, in which the light is produced by the mollusc itself

Hectocotylus

One or more of the male's arms that serve as a modified intromittent organ

Funnel

Ventral region of the foot forming a tubular siphon

Ectocochleate

Type of cephalopod characterized by a well-developed, calcareous external shell

Endocochleate

Type of cephalopod characterized by a reduced internal or absent shell

Body chamber

Newest, largest, anterior-most region of the shell that the animal occupies

Siphuncle

a strand of tissue enclosed in a delicate calcareous tube that extends posteriorly from the visceral mass and passes through the septal perforations to reach the upper end of the shell; functions as the osmotic pump to remove liquid from the chambers and replace it with gas

Cuttlebone

Calcareous, dorsal persistence of the septate phragmocone

Pen

Longitudinal chitinous gladius that the shell is reduced to

Arms

Prehensile, circumoral appendages that are shorter and heavier

Tentacles

Prehensile, circumoral appendages that are longer and retractile

ammonites

• Extinct cephalopods often mistaken for horns by early Romans.

• Folklore: Named after the coiled horn of the Egyptian ram-god Ammon.