In anatomy, an apex (

adjectival form An adjective (abbreviations, abbreviated ) is a word that describes or defines a noun or noun phrase. Its semantic role is to change information given by the noun. Traditionally, adjectives are considered one of the main part of speech, parts of ...

: apical) is part of the

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

of a

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

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

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

scaphopod Scaphopoda (plural scaphopods , from Ancient Greek σκᾰ́φης ''skáphē'' "boat" and πούς ''poús'' "foot"), whose members are also known as tusk shells or tooth shells, are a class (biology), class of shelled Marine life, marine inve ...

, or

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

. 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. Spire ...

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 the initial stage of development for 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 sp ...

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 many gastropods, including snails, 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 ...

. 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 hono ...

'': 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 the long, cone-shaped Exoskeleton, shell belonging to several species of ancient nautiloid cephalopod—the prehistoric ancestors of today's marine cephalopod Mollusca, mollusks, including the cuttlefishes, nautiluses, octopuses an ...

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, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

. The first chamber of the apex is sometimes called the

Bivalves

The apex of a valve of a

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

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 pecking, grasping, and holding (in probing for food, eating, manipulating and ...

. In some species the embryonic shell or

prodissoconch A prodissoconch (meaning first or earliest or original shell) is an embryonic or larval shell which is present in the larva of a bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda ...

may be present at the apex of each of the two valves.

References

Mollusc shells Gastropod anatomy Cephalopod zootomy {{Gastropod-stub