Anatomy of Animals
|Flatworms are nonsegmented, lack a coelom, and have the sac body plan with only one opening. Therefore, if we analyze them according to Table 01, they have a combination of primitive and advanced features. There are three classes of flatworms: one is free living and two are parasitic. The free-living specimen, the planarian, best exemplifies the characteristics of the phylum. Tapeworms and flukes are parasitic with structure reflecting the modifications that occur in parasitic animals. Concomitant with the loss of predation, there is an absence of cephalization; the anterior end notably carries hooks and/or suckers for attachment to the host. The parasite acquires nutrient molecules from the host, and the digestive system is reduced. It is
|covered by a specialized body wall resistant to host digestive juices. The extensive development of the reproductive system, with the production of millions of eggs, may be associated with difficulties in dispersing the species. Figure 06 shows the life cycle of the tapeworm.
- Circulatory - Since the body is flattened, nutrient molecules are easily passed by diffusion from cell to cell.
- Digestive - The digestive organ is tripartite (having 3 branches) and ramifies thorughout the body. It begins with a pharynx (throat), which ejects from the mouth to suck food particles into the digestive organ (see Figure 07(a)).
- Endocrine - Flatworms lack endocrine glands.
- Excretory - Because planarians live in fresh water, which tends to enter the body by osmosis. They have an excretory organ that largely rids the body of excess water. The organ consists of a series of interconnecting canals, which run the length of the body on each side. The beating of cilia in the flame cells (so named because the beating of the cilia reminded some early investigator of the flickering of a flame) keeps the water moving toward the excretory pores (see Figure 07(d)).
- Immune - Immune function requires inter-cellular communication. A number of requisite messenger molecules, such as hormones and neuro-peptides, have been discovered in recent years. Once thought to be restricted to complex multi-celled creatures, similar messengers have been found to influence primitive nervous system and simple non-specific immune cells in low order creatures such as flatworms.
- Musculo-skeletal - Flatworms have three germ layers. The presence of mesoderm not only gives bulk to the animal, it also allows for greater complexity of internal structure. They have well-developed muscles, and their ciliated epidermis allows them to glide along a film of mucus.
- Nervous and Sensory - The planarian has a ladder-type nervous organ (Figure 07(b)). There is a small brain and two lateral nerve cords joined by cross branches. Planarians show good cephalization. Aside from the brain, there are light-sensitive organs (the eyes, which do not form image) and chemosense organs. For primitive invertebrates the eyesight is blurry as shown in diagram 3 of Figure 05c. It improves gradually until the image forming eyesight with the camera-type eye in human.
- Reproductive - Planarians are hermaphroditic animals, which means that they possess both male and female sex organs (Figure 07(c)). The worms practice cross-fertilization; the penis of one is inserted into the genital pore of the other. The fertilized
|eggs hatch in 2-3 weeks as tiny worms. Planarians are capable of regeneration, if a worm is cut crosswise, it usually grows a new head or a new tail as is appropriate.
- Respiratory - The flatworms lack respiratory organ, but since the body is flattened, diffusion alone is adequate for the passage of oxygen from cell to cell.