Benthic Invertebrates

Phylum Porifera

Organisms with no distinct tissues or organs, most sponges are asymmetrical and use choanocytes for filter feeding.

Phylum Cnidaria

Organisms with true tissues, cnidarians are essentially bags made of two cell layers and use cnidocysts to trap prey items.

Phylum Platyhelminthes

Bilaterally symmetrical and triploblastic animals with no body cavity other than the gut, flatworms respire by diffusion and some species are parasitic.

Phylum Annelida

Coelomate animals with a fluid-filled body cavity, annelids have segmented bodies and both a nervous and circulatory system.

Phylum Mollusca

Coelomate animals with a head, foot, and visceral mass, molluscs have a mantle that typically secretes a shell and different feeding habits have influenced their evolution.

Phylum Arthropoda

Arthropods have a stiff cuticle forming an exoskeleton, segmented bodies, jointed appendages, and an open body cavity called a hemocoel.

Phylum Echinodermata

Echinoderms have diversified life styles and typically reproduce through external fertilization, they have a spacious coelom, water-vascular system, and interlocking calcium carbonate plates and spines.

Phylum Chordata

Briefly touching upon tunicates and introducing the Subphylum Cephalochordata within the chordates.

Tunicates

Commonly known as sea squirts, tunicates are marine animals with a simple body structure consisting of a sack with two siphons.

Tunicate Larva

The larval stage of tunicates is free-swimming and exhibits chordate characteristics, including a notochord, dorsal nerve cord, pharyngeal slits, and a post-anal tail.

Metamorphosis

During metamorphosis, the tunicate larva loses its tail and ability to move, and its nervous system disintegrates as it attaches to a hard substrate.

Solitary Tunicates

Some tunicates live a solitary existence after settling, without budding off additional individuals.

Colonial Tunicates

Other tunicates have the ability to bud off additional individuals, establishing a colony.

Cephalochordata

Also known as lancelets or amphioxus, cephalochordates are small, eel-like animals with typical chordate features.

Notochord

Cephalochordates have a muscularized rod called the notochord, which supports the dorsal nerve cord.

Pharyngeal Slits

Cephalochordates have over 100 pharyngeal slits, or gill slits, used to strain food particles from the water.

Myomeres

The musculature of the body in cephalochordates is divided into V-shaped blocks called myomeres.

Post-anal Tail

Cephalochordates possess a post-anal tail, extending beyond the anus.

Brain and Sense Organs

Unlike true vertebrates, cephalochordates have a small and poorly developed brain, as well as underdeveloped sense organs.

Oral Cirri

Water is taken in through the mouth of cephalochordates and filtered by slender projections called oral cirri.

Gill Slits

After passing through the oral cirri, water in cephalochordates is filtered through the pharyngeal slits, while food particles are trapped by mucus.

Digestive System

Cephalochordates have a simple digestive system, with digestion occurring in the intestine after food particles are trapped in a pouch that secretes digestive enzymes.

Circulatory and Excretory Systems

Cephalochordates have a well-developed circulatory system and a simple excretory system.

Reproduction

Cephalochordates have separate sexes, with both males and females having multiple paired gonads. Fertilization occurs externally, and the eggs develop into free-swimming larvae.