Anatomy of Animals

Sponges

		necessary for multicellularity already. Their evolutionary steps are clearly demonstrated from single-celled aquatic protists to colonies and then appear as the collar cells in sponges. A sponge does not have anything in their bodies that can be called tissues or organs. Instead each type of sponge cell is responsible for a different activity to keep the sponge alive. Some sponges grow on rocks and are brightly colored. Sponges often are shaped like vases with a central cavity (Figure 02). Figure 03 shows the anatomy of the sponge. The internal structures are described in the followings.
Figure 02 Sponge [view large image]	Figure 03 Sponge Anatomy [view large image]	Although the terminologies (and hence the functions) for the various systems are in close parallel to those for the human body, the composition and distribution are vastly different.

• Circulatory - The amoeboid cells within the wall act as a circulatory device to transport nutrients from cell to cell. For example, they would pick up some food, move out to the sheets of cells that cover the surface, and feed them.
• Digestive - Sponges are sessile filter feeders. This means that they remain in one place as an adult, and the food they acquire filters through the pores. Microscopic food particles brought by the water are engulfed by the collar cells and are digested by them in food vacuoles or they are passed to the amoeboid cells for digestion.
• Exocrine - There are no endocrine glands in sponges. But they have exocrine glands that release a secretion to the surface. Studies over the last two decades have revealed that sponges produce a surprisingly broad spectrum of biotoxins, some of which are quite potent. A few, such as those of Tedania and Neofibularia can cause painful skin rashes in humans. Not only are those chemicals used to deter predators they are also helpful to prevent infection by microbes and to compete for space with other sessile invertebrates such as ascidians, corals and even other sponges.
• Excretory - The main opening of a sponge is used as an exit to expel waste.
• Immune - Compounds with anti-inflammatory, and antibiotic activity have been identified from many marine sponges.
• Musculo-skeletal - The wall of a sponge contains 3 types of cells. The outer cells are flattened epidermal cells. The inner cells are collar cells with flagella, whose constant movement produces water currents that flow through the pores into the central cavity and out through the upper opening of the body, called the osculum. The mesohyl is a gelatinous layer between the outer body of the sponge and the spongocoel (the inner cavity). The "skeleton" of the sponge is composed of tiny needle-like splinters called spicules (composed of glass or calcium carbonate), a mesh of protein called spongin, or a combination of both.
• Nervous and Sensory - None. But there is some evidence of electrical signals moving along the overall cell membrane.
• Reproductive - Sponges can reproduce sexually or asexually. Sexual reproduction can be either hermaphroditic (organism having both sexes) or gonochoristic (separate sexes). The eggs and the sperms are released into the water, where fertilization occurs. Fertilization results in a zygote, which develops into a ciliated larva that may swim to a new location. Sponges also reproduce by budding, and this process produces whole colonies that can become quite large. Like many less-specialized organisms, sponges are capable of regeneration, or growth of a whole from a small part.

  Germ cells provide the continuity of life between generations. In higher animals (up to roundworms) there is a clear and early separation of germ cells from somatic cell types. For the other species somatic cells can readily become germ cells even in adult organisms such as the buds, and polyps of many invertebrate phyla. For those organisms where there is an established germ line that separates early in development, the germ cells come from the primordial germ cells (PGCs) in the yolk sac and then migrate into the developing gonads (the genital ridges, originally not in the lower part

  Figure 04a Primordial Germ Cells

  of the body) as shown in Figure 04a. Once in the gonad, primordial germ cells continue to divide mitotically, producing millions of potential gametes. The PGCs then undergo meiosis to produce the male and female germ cells, i.e., the sperms and eggs.

  Fertilization is the process whereby two sex cells (gametes) fuse together to create a new individual with genetic potentials derived from both parents. Although the actual details of fertilization vary enormously from species to species, the events of conception generally consist of four major activities: 1. Contact and recognition between sperm and egg. In most instances, this insures that the sperm and egg are of the same species.

  Figure 04b Sperm and Egg [view large image]

  2. Regulation of sperm entry into the egg. Only one sperm can ultimately fertilize the egg. 3. Fusion of the genetic material of sperm and egg. 4. Activation of egg metabolism to start development.

            The shape of the sperm is highly conserved in the evolution of the animal kingdom. Each sperm is known to consist of a haploid nucleus, a propulsion system to move the nucleus, and a sac of enzymes that enable to nucleus to enter the egg. most of the cytoplasm of the sperm has been eliminated during the sperm's maturation as shown in Figure 04b. The structure can be partitioned into the following components (see Figure 04b):
  • Plasma Membrane - In some species such as the parasitic roundworm, the sperm travel by the amoeboid motion of extending the cell membrane.
  • Acrosomal Vesicle - It is derived from the Golgi apparatus and contains enzymes (that digest proteins and complex sugars) to lyse the outer covering of the egg. Recognition between sperm and egg may involve the acrosomal process for some species.
  • Nucleus - The nucleus is very streamlined and its DNA becomes tightly compressed in the head of the sperm.
  • Axoneme - It is the major motor portion of the flagellum emanating from the centriole at the base of the nucleus.
  • Mitochondria - The energy to whip the flagellum and there propel the sperm comes from rings of mitochondria located in the neck region of the sperm.
  • Centriole - The centrosome (Figure 04c) is an area in the cell where microtubles are produced. Within an animal cell centrosome there is a pair of small organelles, the centrioles, each made up of a ring of nine groups of

    microtubules. There are three fused microtubules in each group. The two centrioles are arranged such that one is perpendicular to the other. During animal cell division, the centrosome divides and the centrioles replicate. The result is two centrosomes, each with its own pair of centrioles. The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into the two daughter cells. It is found that the nucleus of the sperm cell is not necessary for the early divisions at the very beginning of development. It is the centrosome (and the centriole within) that is absolutely essential for the initial divisions. There are cases in which a centrosome can actually be created within the egg. This process is called parthenogeneseis, or virgin birth (see more below)

    Figure 04c Centrosome and Centriole [view large image]

            All the material necessary for the beginning of growth and development must be stored in the mature egg (the ovum). Therefore, whereas the sperm has eliminated most of its cytoplasm, the developing egg (the oocyte before it is haploid) not only conserves its material but is actively involved in accumulating more (see Figure 04b for the relative size of sperm and egg). The cytoplasmic trove includes proteins, ribosomes, tRNA, mRNA, and morphogenetic factors (the molecules that direct the differentiation of cells into certain cell types at different locations). The basic components are listed in the followings:
  • Nucleus - In some species such as the sea urchins, the nucleus is already haploid at the time of fertilization. In other species (including many worms and most mammals) the egg nucleus is still diploid and the sperm enters before the meiotic divisions are completed.
  • Plasma Membrane - It must regulate the flow of certain ions during fertilization and must be capable of fusing with the sperm plasma membrane.
  • Vitelline envelope - It forms a fibrous mat above the egg and is essential for the species-specific binding of sperm. It is called zona pelucida in mammals.
  • Cortical Granules - These membrane-bound structures are homologous to the acrosomal vesicle of the sperm. They contain proteins that control the entrance of only one sperm, and support the process of cleavage.
  • Yolk Granule - Yolk is sequestered inside this membrane-bounded sac.
  • Jelly Layer - This glycoprotein meshwork can have numerous functions. Most often, though, it is used to either attract or activate sperm entrance.



		Almost all animal species reproduce sexually, which is believed to have evolved as a mean of strengthening the "fitness" of the species by mixing the genes of two different individuals from meiosis. About 1% of animal species reproduce by partenogenesis (Greek for "virgin birth"), while an even smaller fraction switch between sexual and asexual reproduction (known as cyclical parthenogenesis).
Figure 04d Parthenogenesis Examples [view large image]	Figure 04e Parthenogenesis	Both kinds of parthenogenesis occur most often in low-level species including some fish, amphibians and reptiles (see some examples in Figure 04d). Some experiments try to

induce partenogenesis in chickens and turkeys with limited success. Figure 04e shows the different between normal fertilisation and parthenogenesis. There are actually two ways in parthenogenesis - one method involves sex cell division and recombination, while the other just produces an egg with a full complement of DNA. Many species reproduce parthenogenetically in the absence of a wide pool of potential mates (as alleged in the movies "Jurassic Park"), while some species switch to sexual reproduction only in adverse conditions for increasing the chance of survival. The sexual life of aphids is bizarre : they not only practise cyclical parthenogenesis, but can be born pregnant and males sometimes lack mouths, causing them to die not long after mating. In an addition to their list of anomalies, it is reported in "Nature Online, 17 August 2012" that they may also capture sunlight and use the energy for metabolic purposes, i.e., they have the ability of using photosynthesis to obtain energy similar to plants and algae, as well as certain fungi and bacteria.
• Respiratory - Various cells absorb oxygen from the passing water stream.

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