Coelom, Segmentation & Notochord | Biology for Grade 11 PDF Download

Body Cavity or Coelom

On the basis of coelom, animals can be:

(i) Acoelomate: The animals in which the coelom is absent are called as Acoelomates, for example flatworms. In them the space between ectoderm and endoderm is filled with parenchyma eg. Platyhelminthes.

(ii) Pseudocoelomate: The body cavity is not completely lined with mesoderm. Instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called as pseudocoelom e.g roundworm. 

(iii)  Eucoelomate: The true coelom is a body cavity which arises as a cavity in embryonic mesoderm. In this case, the mesoderm of the embryo provides a cellular lining, called as coelomic epithelium or peritoneum, to the cavity. The coelom is filled with coelomic fluid secreted by the peritoneum. The coelom is found in Arthropods, Molluscs, Annelids, Echinoderms, Hemichordates and Chordates. 

True coelom is of two types:

(a)  Schizocoelom: It develops by the splitting up of mesoderm. It is found in annelids, arthropods and molluscs. Body cavity of arthropods is called heamocoel.      

(b) Enterocoelom: The mesoderm arises from the wall of the embryonic gut or enteron as hollow outgrowths or enterocoelomic pouches. It occurs in Echinoderms, Hemichordates and Chordates.

Segmentation

–    In some animals, the body is externally and internally divides into segments or metameres with serial repetition of atleast some organs. For example, in earthworm, the body shows metameric segmentation and the phenomenon is known as metamerism.

Notochord 

– Notochord is mesodermally derived rod-like structure formed on the mid-dorsal surface during embryonic development in some animals. Animals with notochord are called chordates and those animals which do not form this structure are called non-chordates, e.g., Porifera to Echinoderms or Hemichordates.

Blood Vascular System

Blood vascular system is basically of two types:
Open and Closed

(i)   Open type : In open type, the blood is pumped by the heart into the blood vessels that open into blood spaces (sinuses). There is no capillary system (i.e., most arthropods, some molluscs except cephalopods and tunicates). These sinuses are actually the body cavitites, and are called haemocoel. The pressure of the blood is low; it moves slowly between the tissues, and finally, returns to the heart via the opened veins. In fact, distribution of blood in the tissues is very poorly controlled. The pigments, which carry oxygen, remain dissolved in blood plasma. Body tissues and visceral organs exchange respiratory gases, nutrients and waste products, directly with blood.  

(ii) Closed type : Many invertebrates and all the vertebrates, including humans, have a closed circulatory system.  In closed type, the blood flows around the body through the specific blood vessels. In this system, the same blood regularly circulates in the body under high pressure, and returns back to the heart without leaving the system of tubes. The heart pumps the blood into the aorta, which branches in the body into the arteries, and in the tissues into the arterioles, to form the capillary network. The venules of the capillary network carry the blood back to heart via veins and vena cava. This helps in supplying the nutrients and oxygen to the tissues, and removing waste materials and caron dioxide from it reveals a comparison between open and closed circulatory systems.