Unlike most invertebrate herbivores that may be specialist feeders, including many insects, most land snails are generalists, sampling and assessing a large variety of food items in their path see Speiser Land snails are most often active at night and during damp weather because crawling requires mucus, which is mostly water, and humid air minimizes water evaporation. Once active, snails find food by using the chemoreceptors on their four tentacles, much as mammals use their nose.
One Pennsylvanian species, the flamed disk Anguispira alternata , learned to detour around a barrier to find food that it could smell Atkinson, If you watch some of the larger land snail species, as a snail is travelling it will often have its upper tentacles fully extended, sometimes waving, to sense chemical gradients in the air.
When food is reached, the smaller lower tentacles become more active, often curved downward or touching the food item. The radula is a membrane covered with series of tiny teeth made of chitin, so it is coarse like sandpaper. The shapes of these small teeth are used to help identify some snail species. The radula is drawn over a ridge of cartilage the odontophore , somewhat like a chainsaw chain slides around its bar - though it moves back and forth rather than in a circular motion.
Several bouts of crawling and feeding can occur during an outing. Whelks have a radula on a stalk that can extend beyond the shell and be used to bore into the shells of other molluscs. Through these holes that they have bored they poke the tip of the radula and suck out the flesh of the victim.
Food molecules diffuse or are actively transported into the cells lining the intestine. Food is distributed to the cells of the body by blood.
Blood is pumped by the heart into blood vessels. The blood leaves the vessels and flows into cavities called sinuses. Sinuses surround body organs and extend into all parts of the clam. As the blood moves past the intestine, it picks up food molecules and carries these molecules throughout the clam. The blood eventually enters the pericardial cavity coelom which surrounds the heart.
It then enters the heart though pores, and is then pumped into blood vessels. A circulatory system such as that of the clam in which blood is not always contained within the heart or blood vessels is called an open circulatory system.
Digestive waste leaves the digestive system through the anus. This is located near the excurrent siphon, and the digestive waste is carried out of the clam by water moving outward. The blood circulating through the clam picks up nitrogen waste produced by cells.
The nitrogen waste ammonia is carried into the pericardial cavity. There the kidney removes nitrogen waste from the blood and releases it into water moving out of the clam through the excurrent siphon.
The release of carbon dioxide and acquisition of oxygen is called gas exchange. Gas exchange is the function of the respiratory system. Blood carries carbon dioxide to the gills. There it is released into the water passing over the gills. There also the gills pick up oxygen from the water moving through the clam. Ctenidia are enclosed in a large mantle cavity and are serviced by large blood vessels, each with its own heart associated with it; the mantle has siphonophores that facilitate exchange of water.
Locomotion in cephalopods is facilitated by ejecting a stream of water for propulsion. A pair of nephridia is present within the mantle cavity. Sexual dimorphism is seen in this class of animals. Members of a species mate, and the female then lays the eggs in a secluded and protected niche. Females of some species care for the eggs for an extended period of time and may end up dying during that time period.
Cephalopods such as squids and octopi also produce sepia or a dark ink, which is squirted upon a predator to assist in a quick getaway. Reproduction in cephalopods is different from other mollusks in that the egg hatches to produce a juvenile adult without undergoing the trochophore and veliger larval stages. In the shell-bearing Nautilus spp. These chambers are filled with gas or water to regulate buoyancy. The shell structure in squids and cuttlefish is reduced and is present internally in the form of a squid pen and cuttlefish bone, respectively.
Examples are shown in Figure 7. Figure 7. The a nautilus, b giant cuttlefish, c reef squid, and d blue-ring octopus are all members of the class Cephalopoda. Baecker; credit b: modification of work by Adrian Mohedano; credit c: modification of work by Silke Baron; credit d: modification of work by Angell Williams. Figure 8. Scaphopods are usually buried in sand with the anterior opening exposed to water. These animals bear a single conical shell, which has both ends open.
The head is rudimentary and protrudes out of the posterior end of the shell. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Captaculae serve to catch and manipulate prey. Ctenidia are absent in these animals.
Phylum Mollusca is a large, marine group of invertebrates. Mollusks show a variety of morphological variations within the phylum. This phylum is also distinct in that some members exhibit a calcareous shell as an external means of protection. Some mollusks have evolved a reduced shell. Mollusks are protostomes. The dorsal epidermis in mollusks is modified to form the mantle, which encloses the mantle cavity and visceral organs.
This cavity is quite distinct from the coelomic cavity, which in the adult animal surrounds the heart. Respiration is facilitated by gills known as ctenidia. A chitinous-toothed tongue called the radula is present in most mollusks. Early development in some species occurs via two larval stages: trochophore and veliger.
0コメント