Mollusc | Wikipedia audio article
Channel: wikipedia tts
Date: 04/10/2019

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Mollusca


00:04:46 1 Etymology
00:05:30 2 Definition
00:06:35 3 Diversity
00:09:41 4 Hypothetical ancestral mollusc
00:10:38 4.1 Mantle and mantle cavity
00:11:25 4.2 Shell
00:12:40 4.3 Foot
00:13:31 4.4 Circulatory system
00:14:57 4.5 Respiration
00:15:49 4.6 Eating, digestion, and excretion
00:17:52 4.7 Nervous system
00:19:07 4.8 Reproduction
00:20:58 5 Ecology
00:21:07 5.1 Feeding
00:22:43 6 Classification
00:23:39 7 Evolutionary history
00:23:49 7.1 Fossil record
00:28:03 7.2 Phylogeny
00:31:41 8 Human interaction
00:32:15 8.1 Uses by humans
00:35:49 8.1.1 Bioindicators
00:36:34 8.2 Harmful to humans
00:36:43 8.2.1 Stings and bites
00:38:04 8.2.2 Disease vectors
00:38:50 8.2.3 Pests



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SUMMARY
=======
Mollusca is the second-largest phylum of invertebrate animals after the arthropoda. The members are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied.Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and  anatomical structure, but also in behaviour and  habitat. The phylum is typically divided into 8 or 9 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species.
The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small.
The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic  cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.
Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists.

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