If a Characteristic of Phylum Mollusca is a shell, why don't the Octopus and Squid have shells?

Octopus and Squid are in Class Cephalopoda, but why are they even in the phylum mollusca if they don't have shells?



HELP ASAP!!

If a Characteristic of Phylum Mollusca is a shell, why don't the Octopus and Squid have shells?
In the evolution of the species the shell became more and more a detriment to their hunting abilities as they needed to be faster and able to move about to be the top of the food chain during the late Cambrian period. The shell was internalized in this phylum, and in the case of the cuttlefish it is still present, just as an internal structure, and in the octopus and squid it can be argued that the shell evolved into their beak for their feeding habits which are predatory, and in the steps between nautilus and these 'unshelled' molluscs those that were lighter and more able to move would have been the only ones to be successful predators.



1. The class developed during the Late Cambrian, and were during the Paleozoic and Mesozoic dominant and diverse marine life forms. Small shelly fossils such as Tommotia were previously interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals.[9] Hence, the earliest cephalopod known is Plectronoceras from the Late Cambrian Period. Early cephalopods were at the top of the food chain.

2. Cephalopods , which include the familiar squids and octopus , include species which are the largest known invertebrates (giant squid , up to 20 meters long, including tentacles), the most intelligent, and the fastest swimming aquatic invertebrates (squid ). There are also forms with external shells (nautilus ), and internal hard shells (cuttlefish ). Most of the approximately 650 living species of Cephalopods are active swimmers, however most species of octopus have secondarily assumed a benthic existence.



3. Read the linked article for more.



4. Nautiluses are the most primitive cephalopod group and all have relatively simple, buoyant chambered shells within which the soft body is protected. The high point of nautilus evolution would appear to be during the Paleozoic from about Ordovician and Silurian periods (about 505 to 408 million years ago). During this time giant straight-shelled nautiluses were the only really large animals able to actively swim above the sea floor, sharks were still quite small animals and bony fish hadn't yet become neutrally buoyant. As such these animals must have been the great white sharks of their day, probably eating anything they could find and overpower, but some may also have eaten the swarms of midwater crustaceans rather like whales taking krill today.
Reply:The cephalopod shell was ancestrally external and chambered, as exemplified by the ammonoids and nautiloids, and still possessed by Nautilus today. Other cephalopods, such as cuttlefish, have internalized the shell, the squid have mostly organic chitinous internal shells, and the octopods have lost the shell altogether.



Some snails also lost their shells. The garden slug is a common example. There are also numerous marine slugs.
Reply:all mollusca do not have to possess shells, "mollusca" means soft body and shells are not a mandatory trait....

other key characteristics include bilateral symmetry, body with no cavity, open circulatory system, and a few more characteristics that I don't remember now.. :)

php hosting