In anatomy, an apex ( adjectival form: apical) is part of the shell of a

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

. The apex is the pointed tip (the oldest part) of the shell of a

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

, scaphopod, or

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

. The apex is used in end-blown conches.

Gastropods

The word "apex" is most often used to mean the tip of the

spire A spire is a tall, slender, pointed structure on top of a roof of a building or tower, especially at the summit of church steeples. A spire may have a square, circular, or polygonal plan, with a roughly conical or pyramidal shape. Spires a ...

of the shell of a

. The apex is the first-formed, and therefore the oldest, part of the shell. To be more precise, the apex would usually be where the tip of the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

nic shell or

protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called " ...

is situated, if that is still present in the adult shell (often it is lost or eroded away).

Coiled gastropod shells

The phrase apical whorls, or

, means the whorls that constitute the embryonic shell at the apex of the shell, especially when this is clearly distinguishable from the later whorls of the shell, otherwise known as the

teleoconch The gastropod shell is part of the body of a gastropod or snail, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium storage. Some ...

. Comparison of the apical part and the whole shell of ''

Otukaia kiheiziebisu ''Otukaia kiheiziebisu'' is a species of sea snail, a marine gastropod mollusk in the family Calliostomatidae. The type specimens were collected by amateur conchologist Mr. Kiheizi Oosima in 1938. The specific name ''kiheiziebisu'' is in honou ...

'': File:Calliostoma kiheiziebisu apex.png File:Calliostoma kiheiziebisu shell.png

Limpet-like gastropod shells

Where this feature is present, the space under the apex of a patellate or patelliform (limpet-like) gastropod shell is called the apical cavity.

Scaphopods

The apex of tusk shells is the small, open posterior end, and the opening itself is usually called the apical aperture.

Cephalopods

orthocone An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named '' Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloi ...

cephalopods, the pointed end of the shell is called the apex, and shell growth is away from the apex and toward the

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

. The first chamber of the apex is sometimes called the

Bivalves

The apex of a valve of a

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...

shell is the area around what is most commonly the umbo or

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for fo ...

. In some species the embryonic shell or prodissoconch may be present at the apex of each of the two valves.

References

Mollusc shells Gastropod anatomy Cephalopod zootomy {{Gastropod-stub