Question

Why is octopus categorised in the mollusca group of animals even though its shell is not seen?​

Answer 1

Answer:

It is not necessary for mollusks to have a visible shell because :

Explanation:

Mollusks have soft bodies, and their bodies are not divided into rings like the segmented worms called annelids . Mollusks don't have legs, though some have flexible tentacles for sensing their environment or grabbing things.

Answer 2

Answer:

the easiest separation point between mollusc/mollusk and fish is whether it is a vertebrate. Vertebrate means if it’s an animal that possesses a skeletal structure, vertebral column or backbones, regardless if it’s made of bones or cartilage. Another separate term is craniate which means an animal with a skull. Fishes, including cartilagenous fish (eg. sharks) are craniates and vertebrates, therefore belongs in the phylum Chordata. Molluscs, on the other hand, are animals from the Phylum Mollusca, which are non-craniates and invertebrates. The only exception is octopuses do have a skull-like structure made of cartilage that protect their brain but has no skeletal form.

Mollusca is a big phylum. Quite a number of them have a form of shell made from their mantle to protect their soft boneless bodies. However, not all molluscs have shells and it is definitely not the only thing that classifies an animal as a mollusc. Besides a mantle and two main nerve cords, molluscs may also have a radula (an anatomy structure for feeding), muscular foot/feet, a dorsal visceral mass (a bundle of internal organs on the top side of the body) and a large ceca (“intestine”) and metanephridia (“invertebrate kidneys” but generally called kidney).

An octopus is from a class called Cephalopoda coming from the Mollusca Phylum. All cephalopods are invertebrates that literally look like their name: ‘Cephalo’ is head and ‘Poda’ is foot/feet = “Headfoot” or “Feethead”. Sometimes they have shells like a nautilus while others are absent altogether or have internalised their shell making it look like a backbone but no, it doesn’t count.

Diagram 1. Internal morphology of an octopus. Source: Pinterest. Original source not found.

Diagram 2. Internal morphology of a fish (Largemouth Bass). Source: Australian Museum.

If you compare the two diagram photos, an octopus (also it’s closest comrade the squid) has all it takes to be classed as a mollusc, possessing metanephridia (aka kidney), radula (in form of chitinous tooth called beak), muscular feet and most importantly, a mantle and a lack of backbone unlike a fish which has a proper kidney, proper jaw and teeth (in most), fins instead of feet, no mantle and a skull-backbone combo.