Evolutionary tree is the pictorial and schematic representation of
evolutionary relations among species of living beings in which the trunk (or a preceding branch) represents common ascendants of species and groups of living beings that are distributed along its branches according to hypothesis on their origin. For example, today it is admitted that birds and mammals are two distinct branches of the same preceding reptile branch.
Biology cannot assures a definite evolutionary tree (phylogeny) on the species of living beings that live or have lived in our planet. There are many data to be discovered and many knowledge gaps to be filled. One of the most
promising methods to study phylogeny is the comparison of DNA molecules from different groups of living beings researching similarities and differences in nucleotide sequences that may
indicate more or less relatedness among species.
Poriferans
1. What are poriferans?
The phylum Porifera contains the simplest creatures of the animal kingdom. Sponges are aquatic sessile beings (they are not able to move by themselves and they keep themselves attached to substrates), they do not have tissue diversity and their bodies have pores (feature after which their name comes).
They are multicellular, like all beings of the animal kingdom.
2. What is the way of life of
sponges?
Sponges live exclusively in an aquatic environment and they are attached by their base to a substrate (fixation ground). Sponges are filtering animals, they nourish themselves from nutrients that enter their atrium brought in with water.
3. What is the typical shape of
poriferans?
Sponges have bodies in the form of tubular vases or globes open in the upper extremity. They have an internal central cavity and porous walls. The central cavity is called spongocoel and the opening in the upper extremity is called osculum.
4. How does water move
inside sponges? What is the
function of the pores in these
animals?
Sponges are filtering beings. They make water enter their bodies by their lateral pores. Water then circulates inside the central cavity and exits through the osculum.
5. How do sponges try to
protect themselves against
harm from the environment?
Is that method efficient or
rudimentary?
Sponges can close their pores to avoid the entrance of water into their bodies in the presence of stimulus that may mean danger. This method however is rudimentary but it is actually a
protection attempt against nocent agents.
6. What are the main cells of
which poriferans are made?
Sponges have their outer wall covered by flat cells called pinacocytes and having pores well-delimited by special cells called porocytes. The internal wall is filled with choanocytes, flagellate cells specialized in phagocytosis of food brought to the central cavity; the choanocyte flagella also maintains the water flux inside the sponge.
Between the outer and the inner
cells with amoeboid movement (by pseudopods), the amoebocytes; since they are embedded in connective matrix, amoebocytes move and distribute nutrients to other cells and they also produce spicules that like a primitive skeleton fill the tissue and support the body structure. (Some poriferans have an internal skeleton, an endoskeleton, made of spicules and organic fibers.)
7. Concerning digestion how
are poriferans characterized?
Sponges are different from other animals since they present only
intracellular digestion. They do not have a digestive system nor do they release digestive enzymes in the spongocoel to cause extracellular break down of nutrients.
8. How are animals divided
according to their type of
digestive process?
Apart from sponges, that do not have a digestive cavity where extracellular digestion takes place, all other animals have a digestive system with an internal cavity in which extracellular digestion occurs.
9. How are gases exchanged
in sponges?
The gas exchange in sponges happens by diffusion from the exterior to the
cells that absorb molecular oxygen and liberate carbon dioxide.
10. Do sponges have nervous,
circulatory and excretory
systems?
Sponges do not have a nervous system neither circulatory system nor excretory system.
11. Is reproduction in sponges
sexual or asexual?
Reproduction in sponges can be asexual by budding, gemmation or
fragmentation (regeneration) or sexual with larval stage (a ciliated
amphiblastula larva).
12. What is the evolutionary
advantage of the occurrence
of sperm cells and larval stage
in the life cycle of sponges?
The sexual reproduction in sponges, in addition to contributing to genetic variability, also facilitates the
colonization of farther environments by these beings, since sperm cells and larvae are mobile and can swim in the exterior to compensate the immobility of the adult individual.
13. What is the economic
importance of sponges?
Some chemical substances secreted by sponges have anti-inflammatory,
antibiotic and anti-tumoral activities and they are used in the production of
medicines. Since ancient times the endoskeleton of some sponges has had commercial value, they are used as cleansing implements for baths (bath sponges), to wash animals, objects and so on.