AP Biology        
Notes: Invertebrates

Parazoa

 Phylum Porifera (sponges):          Sponges are sessile with porous bodies and choanocytes

·        Based on both molecular evidence and the morphology                   
  of their choanocytes, sponges represent the lineage
   closest to the colonial choanoflagellates.

·        The germ layers of sponges are loose federations of cells, 
  which are not really tissues because the cells are relatively 
   unspecialized.

·        Sponges are sessile animals that lack nerves or muscles.
  However, individual cells can sense and react to changes
   in the environment.

·        The 9,000 or so species of sponges range in height from 
  about 1 cm to 2 m and most are marine. About 100 species
   live in fresh water.

·        The body of a simple sponge resembles a sac 
  perforated with holes.

·        Water is drawn through the pores into a central cavity, the spongocoel, and flows out through a larger 
   opening, the osculum.    More complex sponges contain branched canals and several oscula.

·        Flagellated choanocytes, or collar cells, lining the spongocoel (internal water chambers) create a flow of 
  water through the sponge with their flagella, and trap food with their collars.

        ·         The body of a sponge consists of two cell layers separated by a gelatinous region, the mesohyl.

        ·        Wandering though the mesohyl are amoebocytes. They take up food from water and from
          choanocytes, digest it, and carry nutrients to other cells. They also secrete tough skeletal fibers 
          within the mesohyl.

·        In some groups of sponges, these fibers are sharp spicules of calcium carbonate or silica.

·        Other sponges produce more flexible fibers from a collagen protein called spongin.  

  Reproduction:   Most sponges are hermaphrodites, with each individual producing both sperm and eggs.

·        Gametes arise from choanocytes or amoebocytes.  The eggs are retained, but sperm are 
  carried out the osculum by the water current. Sperm are drawn into neighboring individuals
  and fertilize eggs in the mesohyl. The zygotes develop into flagellated, swimming larvae that 
  disperse from the parent. When a larva finds a suitable substratum, it develops into a sessile adult.

• They use regeneration not only for repair but also to reproduce asexually from fragments broken off
   a parent sponge.

Radiata

    ·        All animals except sponges belong to the Eumetazoa, the animals with true tissues.

    ·        The oldest eumetazoan clade is the Radiata, animals with radial symmetry and diploblastic 
      embryos.

    ·        The two phyla of Radiata, Cnidaria and Ctenophora, may have had separate origins from
      different protozoan ancestors.

 Phylum Cnidaria: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes

   ( hydras, jellies, sea anemones, and coral animals)

·        They are a diverse group with over 10,000 living species, most of which are marine.

·        The basic cnidarian body plan is a sac with a central digestive compartment, the gastrovascular cavity.

·        This basic body plan has two variations: the sessile polyp and the floating medusa.

·        The cylindrical polyps, such as hydras and sea anemones, adhere to the substratum by the aboral end 
  and extend their tentacles, waiting for prey.

·        Medusas (also called jellies) are flattened, mouth-down versions of polyps that move by drifting 
  passively and by contracting their bell-shaped bodies.

·        Some cnidarians exist only as polyps. Others exist only as medusas. Still others pass sequentially through 
  both a medusa stage and a polyp stage in their life cycle.  

·        Cnidarians are carnivores that use tentacles arranged in a ring around the mouth to capture prey 
   and push the food into the gastrovascular chamber for digestion.

·        Batteries of cnidocytes on the tentacles defend the animal or capture prey. Organelles called
  nematocysts eject a thread that can inject poison into the prey, or stick to or entangle the target.

·        Cells of the epidermis and gastrodermis have bundles of microfilaments arranged into contractile fibers.

·        True muscle tissue appears first in triploblastic animals.

·        When the animal closes its mouth, the gastrovascular cavity acts as a hydrostatic skeleton against 
  which the contractile cells can work. Movements are controlled by a noncentralized nerve net 
  associated with simple sensory receptors that are distributed radially around the body.

Major classes: Hydrozoa, Scyphozoa, and Anthozoa.

Hydrozoans:

·        Most hydrozoans alternate polyp and medusa forms, as in the life cycle of Obelia. The polyp stage,
  often a colony of interconnected polyps, is more conspicuous than the medusas.

·        Hydras, among the few freshwater cnidarians, are unusual members of the class Hydrozoa in that
  they exist only in the polyp form.

·        When environmental conditions are favorable, a hydra reproduces asexually by budding, the formation of outgrowths that pinch off from the parent to live independently.

·        When environmental conditions deteriorate, hydras form resistant zygotes that remain dormant until conditions improve.

 Scyphozoa:

·        The medusas of most species live among the plankton as jellies. Most coastal scyphozoans go 
          through small polyp stages during their life cycle. Jellies that live in the open ocean generally lack
          the sessile polyp.

 Anthozoa:

        ·        They occur only as polyps.   Coral animals live as solitary or colonial forms and secrete 
          a hard external skeleton of calcium carbonate. Each polyp generation builds on the skeletal 
          remains of earlier generations to form skeletons that we call coral.

Phylum Ctenophora: Comb jellies possess rows of ciliary plates and adhesive colloblasts

·        Comb jellies, or ctenophores, superficially resemble
  cnidarian medusas.

·        All of the approximately 100 species are marine.

·        Some species are spherical or ovoid, others are elongated 
  and ribbonlike.

·        Ctenophora means “comb-bearer” and these animals are named 
  for their eight rows of comb-like plates composed of fused cilia.

·        Most comb jellies have a pair of long retractable tentacles. These
  tentacles are armed with adhesive structures (colloblasts) that secrete
   a sticky thread to capture their food.