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Protoconch - Wikipedia

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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 "nucleus". The protoconch may sometimes consist of several whorls, but when this is the case, the whorls show no growth lines.

The whorls of the adult shell, which are formed after the protoconch, are known as the teleoconch. The teleoconch starts forming when the larval gastropod becomes a juvenile, and the protoconch may dissolve. Quite often there is a visible line of demarcation where the protoconch ends and the teleoconch begins, and there may be a noticeable change in sculpture, or a sudden appearance of sculpture at that point.

In some gastropod groups (such as the Architectonicidae), the teleoconch whorls spiral in the opposite direction to the protoconch. In those cases, the shell is called heterostrophic.

In species which have a veliger or swimming larval stage which hatches out of egg capsules, there are two parts to the protoconch. The first part of the protoconch (which is formed within the embryonic egg capsule) is called protoconch 1, while the part that is formed after the larva has hatched is called protoconch 2. There is often a different sculpture or ornamentation on protoconch 1 compared with protoconch 2, and this can be distinguished under the microscope.

The structure of the protoconch has been widely used as a discriminating feature in gastropod systematics. For example, certain nudibranches have an 'inflated' protoconch, which does not grow but houses the growing soft tissues of the larva, and this distinguishes them from other gastropods. ^[1]

The homologous structure in bivalves (clams) is called the prodissoconch.

Examples

Comparison of the whole shell and the protoconch of Atlanta lesueurii:

References

^ Page, Louise (2000). "Inflated Protoconchs and Internally Dissolved, Coiled Protoconchs of Nudibranch Larvae: Different Developmental Trajectories Achieve the Same Morphological Result". Invertebrate Biology. 119 (3): 278–286. doi:10.1111/j.1744-7410.2000.tb00014.x. JSTOR 3227175. Retrieved 20 November 2020.

Cephalopod anatomy

Shell

Types

External	Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone
Internal	Belemnoid guard Cirrate shell Cuttlebone Gladius (pen) Spirula shell Stylet

Features

External	Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus
Internal	Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls

Mantle &
funnel

External anatomy	Dermal structures Fins (wings) Flotation devices ("secondary fins") Funnel–mantle locking apparatus Tail
Internal anatomy	Branchial hearts Ctenidia (gills) Hepatopancreas (digestive gland) Ink sac and ink Needham's sac Nephridia ("kidneys") Nidamental glands Osphradium Pericardial glands

Head &
limbs

Brachial crown	Arms Hectocotylus Tentacles Carpus Dactylus Manus Suckers and hooks
Buccal region	Aptychus Beak Odontophore Radula Spadix and antispadix
Occipital region	Nuchal crest (occipital crest) Nuchal folds (occipital folds) Occipital membrane Olfactory organ Nuchal organ
Other parts	Eyes Statocysts

General

Chromatophores
Photophores
Nervous system
- Squid giant axon
- Squid giant synapse

Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell)

v t e Gastropod anatomy
Shell	Aperture Apex Body whorl Callus Columella Lip Lira Nacre Periostracum Peristome Planispiral Plait Protoconch Sculpture Selenizone Siphonal canal Siphonal notch Spire Stromboid notch Suture Umbilicus Valve Varix Whorl
Other hard parts	Clausilium Love dart Operculum Pearl Radula
External soft parts	Caryophyllidia Caudal mucous pit Cerata Cnidosac Mantle Mentum Notum Parapodium Pneumostome Rhinophore Semper's organ Siphon Suprapedal gland
Internal soft parts	Circulatory system of gastropods Crop Ctenidium Digestive system of gastropods Diverticula Excretory system of gastropods Gastric shield Hancock's organ Hepatopancreas Hypobranchial gland Nephridium Nervous system of gastropods Nidamental gland Odontophore Oesophageal pouch Osphradium Reproductive system of gastropods Respiratory system of gastropods Sensory organs of gastropods
Other	Epiphragm Pseudofeces Torsion Circumesophageal nerve ring Trochophore Tyrian purple Veliger

Category:

Mollusc shells

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