Invertebrate Biology: Annelids, Mollusks, Arthropods

Posted on Nov 25, 2024 in Biology

Leaf Structure

Plants faced dehydration when transitioning to land. Photosynthesis requires light and CO2. Leaf structure adapted to solve this. Angiosperm leaves have architectural adaptations to avoid desiccation. Upper and lower surfaces are covered with transparent epidermal cells coated with a waxy cuticle, reducing water loss. Between is the mesophyll, including palisade cells (columnar, below upper epidermis) for photosynthesis. Lacunar mesophyll (loosely arranged cells with spaces) also aids photosynthesis.

Gas Exchange and Transpiration

In higher plants, water loss (transpiration) and gas exchange (CO2 and O2) are regulated by leaf stomata. Stomata are usually on the lower epidermis to reduce water loss. Guard cells open stomata when turgid (filled with water) and close when flaccid. Cellulosic fibers in guard cells cause this. The wall closest to the opening is thicker. When water enters, thin walls stretch more, bending the guard cells and opening the stoma. Gas exchange and transpiration occur through this opening. When guard cells lose water, the stoma closes, reducing water loss to cuticular transpiration.

Sclerenchyma

Sclerenchyma is composed of dead cells with lignin-thickened walls. Initially alive, they die due to lignin deposition. Sclerenchyma provides strength and support to plants with secondary growth. Two types: fibers (elongated) and sclereids (isodiametric).

Primary Structure of Roots and Stems

Plants grow in length (primary) and thickness (secondary). Monocots usually have only primary growth; dicots have both. Differences exist between monocot and dicot root and stem structures.

Primary Monocot Root

  • Epidermal Zone: Suberized cells.
  • Cortical Zone: Cortical parenchyma with intercellular spaces and plasmodesmata. Endodermis is compact with U-shaped suberized cells.

Primary Dicot Root

  • Epidermal Zone: Single layer.
  • Cortical Zone: Storage tissue with endodermis and Casparian strips.

Secondary Dicot Root

  • Vascular cambium produces xylem and phloem.
  • Phellogen produces cork and phelloderm.
  • Multiseriate medullary rays and heterogeneous xylem.

Primary Monocot Stem

  • Cutinized surface layer.
  • Cortical zone with living cells.
  • Scattered vascular bundles with sclerenchyma fibers.
  • No secondary growth.

Primary Dicot Stem

  • Epidermal layer.
  • Cortical parenchyma with chloroplasts.
  • Central cylinder with pith, vascular bundles, and medullary rays.
  • Xylem develops centrifugally.

Annelida

Annelids are segmented worms with a cylindrical body. Segmentation is complete, external, and internal. They are triploblastic, protostome coelomates. The body is bilaterally symmetrical and covered with a moist cuticle. They have setae for locomotion (except leeches) and longitudinal and circular muscles. Hydrostatic skeleton provides support.

Nutrition

Complete digestive system with mouth, anus, pharynx, esophagus, crop, gizzard, and intestine with typhlosole. Leeches have suckers for feeding on blood, using anticoagulants.

Physiology

Four cerebral ganglia. Metabolic wastes removed by metanephridia. Gas exchange through skin or gills (polychaetes). Closed circulatory system with dorsal and ventral vessels. Oligochaetes and leeches are monoecious; polychaetes are dioecious with trochophore larvae.

Mollusca

Mollusks are important invertebrates for food and raw materials. Some snails (Biomphalaria) transmit diseases.

  • Monoplacophora: Single shell (Neopilina).
  • Polyplacophora: Shell with plates (Chiton).
  • Scaphopoda: Tusk-shaped shell (Dentalium).
  • Gastropoda: Shell or shell-less (slugs, snails).
  • Pelecypoda: Bivalves (mussels, oysters).
  • Cephalopoda: Advanced mollusks (octopus, squid).

Habitat

Soil, freshwater, and marine environments.

Structure

Soft body, unsegmented, bilateral symmetry, with or without shell. Body divided into head, visceral mass, and foot.

Physiology

Extracellular digestion. Ganglionic nervous system. Modified nephridia for excretion. Diverse respiration (skin, mantle, gills). Open circulatory system (except cephalopods). Sexual reproduction, some hermaphroditic, with direct or indirect development.

Arthropoda

Most numerous group, characterized by jointed appendages and exoskeleton. Important for pollination, food, and disease transmission.

Classes

  • Insects: Bees, flies, etc.
  • Crustaceans: Crabs, shrimps, etc.
  • Arachnida: Spiders, scorpions.
  • Centipedes and Millipedes.

Habitat

Diverse habitats: soil, water, living organisms.

Structure

Bilateral symmetry, segmentation, chitinous exoskeleton, jointed appendages.

Physiology

Extracellular digestion with accessory glands. Ganglionic nervous system with sensory organs. Malpighian tubules or green glands for excretion. Diverse respiration (skin, tracheae, gills). Open circulatory system. Sexual reproduction with metamorphosis.

Metamorphosis

  • Holometabolous: Complete metamorphosis.
  • Hemimetabolous: Partial metamorphosis.
  • Ametabolous: No metamorphosis.