The creatures of the Earth have different sizes. All of vertebrata, mollusk, worms, insects, belong to the animal kingdom. But we see that their individual kinds show extreme variations in size. The largest animals, such as whales and giraffes belong to the class of Mammalia, and the smallest ones, i.e. insects and Acarina belong to Arthropoda. Largeness and smallness are relative qualities. A thing, on its own, cannot be large or small, and only by comparison with other things, becomes large or small. If we only observed the insects of the world, we couldn't tell that they are small creatures, but when we see a fly resting beside a horse, we become surprised at the great difference of their size, and the question occurs to us: Why are insects smaller than other creatures? "Because of the limited weight of hard exoskeleton, the size of no arthropoda creature encroaches a determined extent." This has been the answer of zoologists and entomologists to the above question. But the answer isn't satisfying even for themselves. Because the body of many insects is soft, but they yet are very small, and against those are crayfishes whose exoskeleton is very hard, yet are greatly larger than insects! Entomologists compare the body structure of an insect with that of a large animal such as a giraffe. They are searching for factor in bodies of insect, which prevent their getting big. But till now they have had little success. For finding an appropriate answer we must look in another direction. In other words, we must instead of searching for the preventive factor of growth of insect, search within the system in the body of the giraffe, which the insects lack, and which contributes to greater size of the body. The head, which carries the brain, is the most important part of any creature's body. The brain is important and sensitive, and must receive enough water and food all the time. Providing necessary water and materials to the brain is among the primary responsibilities of circulatory system. The heart, like a pump, turns the blood, which carries water, and nutritive substances and the vessels carry the blood to the brain, by use of other organs, which act like tubes. Suppose we want to send water from a tank, which is placed on the lower part of a structure to upper floors for usage of resident of the structure. But if we do not use tubes (plumbing) and simply tried to carry on the job only with a pump, it would be a difficult task. The pump can not carry the water to regarded points of the upper floors without tubes and plumbing, and only to a limited extent can the pump shoot it upwards. In this case, the water not only won't reach to desired points of upper floors of the structure, but also at the lower floors, instead of arriving at its desired points (bath and kitchen) it will flood into all the rooms and walls. Without plumbing tubes, we couldn't make high structures functional, and we would be convinced to construct low ones in order to distribute the water for its residents. Within the body of the giraffe, there is not only a strong pump (heart), but also many "plumbing" tubes (arteries, veins, capillaries), so its body can convey blood to the highest points where the brain is located, and to other organs and systems, easily as well. Any organ of the body of a giraffe can receive as much blood as it needs. When the animal eats food, the blood is succinctly guided to its digestive system, operates as the feeding organ. Any cell of the body of the giraffe can receive necessary amounts of water and other substances from nearest capillary systems and send redundant and unnecessary substances to be discarded, through the same system. But the body structure of an insect possesses an entirely different system. There is no "plumbing" to distribute the "water", but only a pump (heart), which carries the blood from the dorsal and pushes it forward. There isn't any upper floor in this structure, and at the lower rooms, the water, instead of flowing in tubes, fills all the rooms, and the residents are basically flooded in water. Individuals in the lower floors of an insect, not only receive food from the "water", but also pour the unnecessary substances into it. The insects have an open circulatory system. This means that there are no vessels or "plumbing" within their body, and the blood moves openly through it. There, every place is full of blood, and organs and systems of the body are "drowning" in it. They receive water and necessary substances from it, and pour the unnecessary excrament into it. The heart, for the lack of vessels, cannot convey the blood to far distances and high points, so all the systems of the body have to gather around the heart, so as not to suffer the shortage of water and necessary substances. The circulatory systems of insects have little authority on the rate of distributing the blood to different organs, and organs which are near the heart, receive more necessary materials and enjoy preferable conditions. The brain, being the most important, is located in front of the aorta, where the blood first emerges from the heart. The heart always takes the blood from the back of the body and pushes it forward towards the head. If the brain of insect was at a high point, like the giraffe's brain, the heart couldn't send blood to it for lack of vessels, so the brain and other organs of insect have to be near the heart. In other words, the size of the insect must be small. In this manner, it is obvious that any creature wanting to grow in size, must have the necessities of distributing the blood, and because the insects lack such means, they cannot grow in size. In other words, the insects, by lacking the "plumbing" vessels, have small sizes, and the giraffes having so many vessels can then become comparatively large. On the whole it can be said that any creature which has a more complete circulatory system, will become larger than others. To help make the matter more clear, it is noteworthy to compare the circulatory system and size of some other creatures also. There are species in the worm groups which, on the whole, lack the circulatory system and are very small and microscopic, i.e. Nematoda and Bryozoa. Some of worms have a simple circulatory system consisting of some linear vessels and a small heart. These species can make themselves a little larger, such as Phoronidea, Sipunculoidea and Brachiopoda. Among the worms, only the segmented ones have an advanced circulatory system, consisting of linear and partial vessels, some hearts and capillaries. Their blood circulation is closed. The segmented worms, having such a circulatory system are the largest species among worms, i. e. Rhinodrilus Fafneri which reaches 210cm long and 2.5cm diameter, and Eunice Gigantea which is 3m, long. In the Mollusca group, we see a variety of circulatory systems, both open and closed. The individuals belonging to open circulatory system, and having few vessels and a simple heart, such as Amphineura and Gastropoda are smaller. But individuals whose circulatory system is more advanced, such as Bivalves, are of a larger size. Finally, Cephalopoda, who are the most developed ones of Mollusca, and have a closed circulatory system with many vessels and heart chambers, are larger than other Molluscas, such as large Octopuses and Architeuthis, which have a body of 2.5m long and arms as longs 12-18m. All members of the Arthropods group have an open circulatory system, so they can not grow very much. The extent of the evolutionary process, however, is not the same among all individuals of this group. Among the arthropods, Crayfishes are the largest species, because their circulatory system, in addition to their hear,t has many arteries and the heart can guide the blood to relatively far distances within them. But because of lacking capillaries and veins, the blood is distributed in its coelum "body cavity". Also, the circulatory system is open. The Circulatory system of Scorpions and some other Arthropods has fewer arteries, so they are smaller than Crayfishes. All insects lack vessels, and only have a Vaisseau Dorsal, which consists of some ventriculites and end in a short aorta. So insects must remain a small relative size. Among insects, those which have longer Vaisseau Dorsal, such as Cockroaches and Grasshopers are larger than ones whose Vaisseau Dorsal is shorter, having less ventriculites, i.e. Coccidaes. The size of the smallest insects is less than the greatest Monocells, i.e. 0.25mm. But these insects are not the smallest Arthropods. Acarina are the smallest ones. Parasite Vegetable Acarias are even smaller that 0.1mm. For observing Acarias of Eryophidae group, magnifying instruments are needed and used. So these small Acarias must have some simpler circulatory system than even insects. Larger Acarias, like insects, have a Vassea Dorsal and heart. But small Acaria lack such organs and their circulation is made only by body muscles and movements of internal organs such as the digestive system. Among Vertebrata, the ones who have a more complete circulatory system, are the larger creatures, i.e. Mammalia, whose circulatory system is the most complex and complete, so they become the larger species such as the whale, elephant and giraffe. So as a whole we conclude that any creature living on earth, establishes a direct relationship between its circulatory system and its relative size. No creature on Earth is an exception to this principle.

Compare these animals considering their body size and structure of blood circulation system with each other.

 

ARTHROPOD	Structure of circulatory system	Body size
Acaria	Lack such organs and their circulation is made only by body muscles and movements of internal organs such as the digestive system.	Very small and microscopic.   Eryophyes (gall mite) which 0.1 mm long.
Insects	Circulatory system have a vessel dorsal and heart	Small
Crustacean	circulatory system in addition to their heart�s has many arteries	Largest species among arthropods

  

WORMS	Structure of circulatory system	Body size
Nematoda Bryozoa	On the whole, lack the circulatory system	Very small and microscopic
Phoronidea Sipunculoidea Brachiopoda	These worms have a simple circulatory system consisting of some linear vessels and a small heart.	Little larger
Segmented worms	These worms have an advanced circulatory system, consisting of linear and partial vessels, some hearts and capillaries. Their blood circulation is closed.	Largest species among worms.   Rhinodrilus Fafneri which reaches 210cm long and 2.5cm diameter, and Eunice Gigantea which is 3m, long.

 

Mollusca	Structure of circulatory system	Body size
Amphineura Gastropoda	Open circulatory system, few vessels and a simple heart.	Small
Bivalves	circulatory system is more advanced	larger size
Cephalopoda	Cephalopoda, who are the most developed ones of Mollusca, and have a closed circulatory system with many vessels and heart chambers	Largest species among Mollusca   Octopuses and Architeuthis, which have a body of 2.5m long and arms as longs 12-18m.

  

Land vertebrata	Structure of circulatory system	Body size
Amphibians	Larvas have the 2-chambered hearts. Larva usually aquatic.   Adults have the 3-chambered hearts.	small
Reptiles	Circulatory system is imperfect and their dark blood is mixed together with the bright one, because their heart is 3-chambered. An incomplete wall between their ventricle	Reptiles can not hold their head upward, and have to creep on the ground and have shorter legs,
Birds	After mammals, birds have the most developed circulatory system. Their heart is also 4-chambered	Larger of reptiles, long necks and legs
Mammals	Mammals have the most developed circulatory system and, 4-chambered heart	Largest land animals. Mammals have the longest necks and legs.

So as a whole we conclude that any creature living on earth, establishes a direct relationship between its circulatory system and its relative size. No creature on Earth is an exception to this principle. 

Ramin Amir mardfar

 Ramin And Prof. Dr. Simon Maddrell

 Ramin And Prof. Dr. Dominique Adriaens