............................. KINGDOM ANIMALIA (METAZOAS) I. The non-arthropod invertebrate

Introduction. The animals are certainly the most complex living beings and, therefore, the most interesting ones to study about. Thanks to having nerve cells and muscle cells they can act very quickly and this makes them astonishing. The 90% of animal species are invertebrates, which means that they are animals without an internal articulated skeleton that protects nerve cord, as happens in with our backbone. The group of the invertebrates animals is actually a completely artificial group, meaning that it was human who invented this group, because it helps to differentiate vertebrates animals such as ourselves. Actually it is a group of animals that have nothing to do with each other. For example a sponge, a snail and an ant, it is clear that they have very little in common. From this, it has been inferred that the current invertebrate groups had very different origins, which means that we must go back to very simple forms of the first multicellular organisms to find their common ancestors. This explains why they are so different from each other.

In the invertebrates we can distinguish two major groups, which are the arthropods, which are the invertebrates that have the articulated external skeleton, such as insects and crustaceans have, and the non-arthropods invertebrate, which is the rest of the animals. Although arthropods are present as much in aquatic environments as in the land, the majority of the non-arthropod invertebrates live only in the aquatic or very humid environments. If we think that life appeared in the aquatic environment, we understand that non-arthropod inverebrates are the forms, that due to its characteristics, have not been able to colonize the terrestrial environment. After studying in detail, the reasons are understood.


Read the explanations on the general characteristics of sponges, cnidarians and the annelids and perform Multiple Choice Test 16.1 and Relate pictures with names 16.1. Then read the text on the flatworms, molluscs and echinoderms and perform Multiple Choice Test 16.2, Relate pictures with names 16.2 and Crossword 16.


1. The animals or Metazoas. Are the multicellular organisms with specialised tissues and with heterotrophic nutrition, which means that they feed on other organisms. They have a great sensivity due to the nerve tissue and can perform movements (the majority can even move) thanks to the muscle tissue. These two tissues are unique for animals. We can distinguish two groups:

• Invertebrates. Are the animals without vertebral column. Includes: Poriferas or sponges, Cniadrias, Annelidas, Platyhelminthes, Molluscas, Echinodermatas and Arthropodas .

• Vertebrates. Are the animals with vertebral column. Includes: Fishes, Amphibians, Reptiles, Birds and Mammals .

2 . Poriferas or sponges. Are the aquatic invertebrates that have their body pierced by a large number of microscopic pores (inhalants pores) through which the water enters to the internal cavity (atrial cavity), from where exits through pores visible to the naked eye (osculums). In the atrial cavity there are some flagellated cells (choanocytes) capable to capture the food particles that contain water (therefore are filter organisms). Some sponges contend small internal spines (spicules) and others, such as bath sponges, have an internal skeletal substance of nature cornea (spongin). The sponges which we use in the bathroom are actually skeletons of spongins that are left to desintegrate from the rest of the body.

3 . Cniadrias. Are the aquatic invertebrates that have form of the sac of very thin walls, with tentacles surrounding the opening (mouth) and with abundant stining cells (cnidoblasts) on its skin. With these cells they paralyze their preys and then they introduce them in its intern digestive cavity (gastrovascular cavity) where they are digested. They have two different forms: the polyp form (a form of sac attached to the bottom and with the mouth directed upwards) and the jellyfish form (a form of sac with the mouth directed downward and with free swimming lifestyle). In some species de polyp form generates the jellyfish form and vice versa. In the others there is only one of those two forms.

 


4. Anelidas. Are the invertebrates that have an elongated body, are soft and cylindrical, they are formed by several rings (segments) which are almost equal, that means they have the shape of a worm. Its surface is covered with an elastic cuticle and may have very fine hairs (setae).

The anelidas are classified into three groups:
The classification of the Annelidas
Oligochaetes. Are those that have few setae. For example the earthworm. Polychaetes. Are those that have many setae. For example the nereis or worms that are used for fishing in the sea. Hirudineas. Are those that have suction cups at their ends. For example the leeches.


5. Platyhelminthes. They are invertebrates with form of a levelled and wide worm .They can have segments such as Taenia present or not, as the planarian. The Taenia or Tapeworm is a parasite of about 2 or 3 meters long that can live in the small intestine of animals, including humans.The infestation occurs by eating raw meat (ham, "cured meat", etc.) of animals that Taenia encysted larvae contend.The planarians live in fresh water and at sea, are carnivorous and have about one centimeter in length.
Relate pictures with names (Sponges and Cniadrias)
Complete sentences about sponges (access to the M. Alós WEB)
Multiple Choice Test about Cniadrias (access to the M. Alós WEB)

6 . Molluscas. Are the invertebrates with soft bodies, non-segmented, often divided into head, visceral mass and foot. In many of them the visceral mass is surrounded by a mantle that produces a calcium carbonate shell. Furthermore, the mantle forms a cavity called the mantle cavity or the pallial cavity where the respiratory system is (lung in terrestrial species and gills in aquatic species).
Molluscas are classified as:
The classification of Molluscas

Gastropods. They have a muscular foot capable of sliding over all types of surfaces and a shell formed of a single piece as the land snail or lack of it as the slugs. The shell may be flat as barnacles or formed as a spiral, as in snails.
The breathing may be through gills, like marine snails, or through lungs, as in the land snail, which is hermaphrodite.

Bivalves. They have a shell formed by two articulated pieces called valve.
They do not have a differentiated head and the foot have a shovel shape or is greatly reduced and has a gland that secretes the byssus, a substance that through solidifying allows them to stick to the rocks. They breathe through laminar gills which also serve to retain particles (filter feeders) that enters through a tube (inhalant siphon) and goes out through the other (siphon exhalant). For example clams and cockles.

Cephalopods.They have a foot divided into 8 or 10 tentacles provided with suction cups around the mouth. They have a well differentiated head with two big eyes. The mantle has a form of a sac and the water that enters into it can be expelled with force use through a tube (siphon) in a flow that allow swimming response. They may lack of the shell like octopuses, have internal shell such as cuttlefish and squid, or external as nautilus.

 

 

 


7. Echinodermatas. Are the invertebrates, exclusively marine, that under its skin (epidermis) possess a calcareous skeleton composed of welded or independent plates or composed of small and isolated spicules. The plates can serve to support long spines.

They present a set of pipes called ambulacral system, filled with a fluid similar to the seawater, and a series of suction tubes finished in suckres which serve to fix and move and are called ambulacral feet. They have pentagonal symmetry (very evident in starfish by presenting 5 arms).

They reproduce sexually. They present separate sexes, meaning there are male or female individuals.
Fertilization takes place in the sea (external fertilization) where a swimming larva (planktonic larvae) comes out of the egg and afterwards result in an adult walker (the adult is therefore a benthonic organism).


The Echinodermatas are classified into five different groups, which are:

The classification of Echinodermatas

Sea urchins. They have a skeleton made of calcareous plates that are welded together, forming a globular structure which supports long spines. They are herbivores.

Starfishes. They have five arms inflexible and may have thorns. They are carnivorous and can evaginate the stomach to digest the prey.

Holothurians. These are called "sea sausages" in reference to its shape. They feed on the little organisms that contain sand which they ingest.

Brittle stars. They are like starfishes but with very flexible arms that move quickly. They live under rocks on the seabed.

Comatulidas. They have five arms which are split into two, with them many species can swim, but only for a very short time. They have bright colors.
 
Multiple Choice Test about Molluscas (access to the M. Alós WEB)
Relate pictures with names (Molluscas)
Relate pictures with names (Echinodermatas)
Crossword
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