Why do cephalopods have a radula

Form and shape of molluscan radular teeth are commonly exclusive to a species or genus. Each row of radula teeth consists of one rhachidian tooth followed by one or more lateral teeth on each side, and then one or more marginal teeth. SEM photographs were taken to further study the radula in detail.

Several methods are available in regards to preparing and mounting the radula of several mollusc species for SEM observations, but only one in specific detail about cephalopods. Zheng was the primary resource of this paper. Radulae or buccal cavities were retrieved from three cephalopods. Vampyroteuthis infernalis was given by Dr. The seriation can involve either asymmetrical left drawing or symmetrical right drawing and the scanning electron micrograph rhachidian teeth.

Rachidian teeth. Left - Octopus vulgaris. Drawing modified from Nixon, Middle - Octopus macropus. Drawing modifed from Nixon Right - Octopus sp. Scanning electron micrograph by Mark Norman. The radulas of Nautilus spp. A total of 13 elements is present. The illustration on the right has attempted to clarify the arrangement by the elimination of the first marginal tooth from a number of the rows.

Chitons crawl along rocks looking for food usually algae. A chiton uses its radula tongue to scrape algae off rocks. It also has very hard teeth that are also used to scrape algae of rocks. These teeth are hard enough to etch glass! Embedded within their shells are primitive "eyes" that can detect light. Chitons are very, very slow moving. During a year, a chiton may move only ten feet! The Bivalves The bivalves class Bivalvia are very well known.

They include clams, mussels, oysters and scallops. All bivalves have two shells the name means "two shells" , and there are about 15, species. Most bivalves do not have radula because they eat by filtering water through their gills to obtain organic particles.

These animals lack a calcareous shell but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia excretory organs. The morphology of the shell and the underlying animal can vary from circular to ovate. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals.

The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in Today, scientists have identified nearly two dozen extant species.

These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators. Respiration is facilitated by ctenidia gills that are present ventrally.

These animals possess a radula that is modified for scraping. Eyespots are absent in these animals. A single pair of nephridia for excretion is present. Figure 3. These mussels, found in the intertidal zone in Cornwall, England, are bivalves.

Members of this class are found in marine as well as freshwater habitats. The overall morphology is laterally flattened, and the head region is poorly developed.

Eyespots and statocysts may be absent in some species. These animals are suspension feeders—they eat material, such as plankton, that is suspended in the water around them.

Due to their diet, this class of mollusks lacks a radula. Respiration is facilitated by a pair of ctenidia, whereas excretion and osmoregulation are brought about by a pair of nephridia. Bivalves often possess a large mantle cavity. In some species, the posterior edges of the mantle may fuse to form two siphons that serve to take in and exude water. One of the functions of the mantle is to secrete the shell.

This property has been commercially exploited to produce pearls. Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell Figure 4. Shells may be planospiral like a garden hose wound up , commonly seen in garden snails, or conispiral , like a spiral staircase , commonly seen in marine conches. Figure 4. The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling Figure 5.

Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species.