Invertebrate Biology: Porifera, Cnidaria, and Mollusca

Exam Tip: When identifying sponge structures under a microscope, remember that in syconoid and leuconoid sponges, the spongocoel is lined by pinacocytes, NOT choanocytes. The choanocytes are restricted to the specialized chambers.

Taxonomic Classification and Reproduction

Sponges are further divided into three taxa based on their skeleton: Calcarea (calcareous spicules), Hexactinellida (six-rayed siliceous "glass" spicules), and Demospongiae (spongin fibers and/or siliceous spicules).

Reproduction can be sexual or asexual. In sexual reproduction, sperm is released into the water and captured by another sponge's choanocytes, which then transport the sperm to an egg in the mesohyl. Asexual reproduction often involves the formation of gemmules—tough, overwintering clusters of cells protected by a spongin casing and a specialized opening called a micropyle.

Phylum Cnidaria: Tissues and Stinging Cells

Unlike sponges, Cnidarians possess true tissues, including an epidermis and a gastrodermis, separated by a jelly-like mesoglea. They utilize a gastrovascular cavity (GVC), which serves as an incomplete digestive tract with a single opening.

The Cnidocyte and Feeding

Cnidarians are defined by their specialized stinging cells called cnidocytes, which contain nematocysts. These structures are used to paralyze prey or defend against predators. Feeding is facilitated by tentacles surrounding the mouth (often located on a structure called the manubrium).

Life Cycles: Polyps and Medusae

Cnidarians exhibit two primary body forms that often alternate in their life cycles:

• Polyp: A sessile, cylindrical form (e.g., Hydra or corals) that often reproduces asexually via budding.
• Medusa: A mobile, umbrella-shaped form (e.g., jellyfish) involved in sexual reproduction.

In the class Hydrozoa (such as Obelia), the life cycle often involves a transition from a polyp stage to a medusa stage. In contrast, Anthozoa (such as sea anemones) exist only in the polyp stage.

Phylum Mollusca: Complex Body Plans

Mollusks are highly diverse invertebrates characterized by a tripartite body plan: a mantle (which secretes the shell), a muscular foot for locomotion, and a visceral mass containing the internal organs.

Major Classes of Mollusca

The phylum is divided into several distinct classes with varying anatomical specializations:

1. Polyplacophora (Chitons) Chitons are marine grazers characterized by a shell composed of 7 or 8 overlapping plates (valves). They possess a large, muscular ventral foot for clinging to rocks and a mantle that often includes a fleshy region called a girdle. They use a specialized, iron-hardened radula to scrape algae from surfaces.

2. Gastropoda (Snails and Slugs) Gastropods exhibit complex internal anatomy. Many terrestrial species, like Helix, are monoecious (having both male and female organs) and utilize a unique mating ritual involving "love darts" to inject hormones into their partner. This increases the success rate of fertilization. Key structures include:

• Ctenidia: Gills used for gas exchange.
• Mantle Cavity: Contains the pneumostome (breathing opening) in terrestrial species.
• Bursa copulatrix: A structure involved in the processing of received spermatophores.

3. Scaphopoda (Tusk Shells) Scaphopods possess a single, uncoiled, tube-like shell that is open at both ends. They live partially buried in sediment and use long, sticky tentacles called captacula to gather food.

Physiological Systems

Mollusks possess a circulatory system involving a heart and hemolymph. Their digestive tract includes a buccal mass and salivary glands to assist in processing food.

Summary Table: Comparative Invertebrate Anatomy