Protostomes 1 - Overview and Spiralia/Lophotrochozoans

This set covers the classification and biological characteristics of protostomes, specifically focusing on the body cavities and major groups within the Lophotrochozoan clade.

Protostomes

Triploblastic bilaterians where the blastopore develops into the mouth during gastrulation.

Anterior brain

A major derived trait of protostomes that surrounds the entrance to the digestive tract.

Ventral nervous system

A derived trait of protostomes consisting of paired or fused longitudinal nerve cords.

Acoelomate

An animal with no fluid-filled cavity; instead, the internal cavity is full of cells forming the mesenchyme.

Pseudocoelomate

An animal with an internal cavity full of fluid ($\text{pseudocoel}$) in which organs are suspended and muscles are located on the outside.

Coelomate

An animal with an internal cavity ($\text{coelom}$) lined with a layer of tissue called peritoneum, which also covers the organs.

Peritoneum

A layer of tissue that lines the coelom and covers internal organs.

Hemocoel

A "blood chamber" in arthropods that is part of the open circulatory system, allowing for free circulation within the body.

Chaetognathes

Predatory, usually planktonic, marine "arrow worms" whose classification within protostomes remains unclear.

Lophotrochozoa

One of the two main clades of protostomes, many of which possess a trochophore larva or a lophophore.

Ecdysozoa

One of the two main clades of protostomes containing animals that must molt their external cuticle to grow.

Lophophore

A horseshoe-shaped crown of ciliated tentacles around the mouth used for filter-feeding, found in sessile groups like Brachiopods and Bryozoans.

Trochophore

A type of free-living larva characterized by bands of cilia, most notably found in Annelids and Molluscs.

Ecdysis

The process of molting or shedding an external cuticle and replacing it with a larger one, which evolved approximately $500$Mya.

Platyhelminthes

Acoelomate animals known as flatworms that are dorsoventrally flattened and lack a gas transport system.

Rotifera

Tiny ($50$–$500$$\mu\text{m}$) mostly freshwater animals that have specialized internal organs, a complete gut, and are highly resistant to ionizing radiation.

Bdelloid rotifers

A group of rotifers consisting only of females whose eggs develop without fertilization.

Bryozoans

Also called "moss animals," these are aquatic, mostly marine sessile colonies made of small individuals called zooids.

Cycliophora

Microscopic marine invertebrates discovered in 1995 that live on the mouthparts of lobsters.

Nemertea

Commonly known as ribbon worms, they possess a muscular proboscis used for feeding.

Brachiopods

Solitary marine animals with two-part shells connected by a ligament, most of which have a stalk to attach to a substrate.

Annelida

Segmented worms restricted to aquatic or moist habitats where the coelom in each segment is isolated from other segments.

Mollusca

A diverse group of protostomes that includes the largest known invertebrates, many of which have a calcareous shell and trochophore larvae.