Protostomes are bilaterally symmetrical, triploblastic animals. They differ from deuterostomes in several important ways. Cleavage is spiral, rather than radial. Cleavage is also determinate, meaning that each cell has a specific fate from the earliest stages of development. The blastopore develops into the mouth (or into the single gut opening if the organism has an incomplete digestive system). Mesoderm develops from a single blastomere located near the blastopore. Protostomes are far more numerous than deuterostomes, mainly due to the inclusion of the arthropods (although mollusc species alone outnumber deuterostome species).

Lophotrochozoans

Lophotrochozoans are one of the two major clades of deuterostomes (Ecdysozoa) is the other. The word lophotrochozoa is a contraction of two words – lophophore and trochophore. The lophophore is a feeding structure that generally consists of ciliated tentacles. The trochophore is a particular type of larva that is free-swimming due to a band of cilia that girdles it. Some lophotrochozoans have one of these structures; others have neither and are placed within the group due to molecular homology. Lophotrochozoans include Phylum Platyhelminthes, Phylum Rotifera, Phylum Mollusca, Phylum Annelida, and a number of other phyla.

Platyhelminths

Flatworms, or platyhelminths, belong to Phylum Platyhelminthes. They are bilaterally symmetrical, triploblastic, acoelomate animals. Their body plan is relatively simple. They are cephalized, and usually have a single brain located in the head region. The brain communicates with the rest of the body via two ventral nerve cords that run the length of the body.

Flatworms generally have a two-way digestive system, with a single opening that functions as both mouth and anus, although some species have a complete digestive system with an anus (or sometimes, more than one anus). Tapeworms have lost their digestive system altogether and rely on absorption to obtain nutrients. Flatworms lack specialized respiratory and circulatory systems, relying on diffusion for transport of oxygen, carbon dioxide, and nutrients.

Platyhelminths exhibit several modes of reproduction. Some species of planarian have never been observed to reproduce sexually. Other flatworms are dioecious, with separate male and female individuals. However, most flatworms are hermaphrodites, producing both eggs and sperm.

There are about 29,000 flatworm species. Some are free-living, inhabiting marine, freshwater, or terrestrial ecosystems. Others are parasitic, and have life cycles involving one, two, or three hosts. Many are significant human parasites, including tapeworms, liver flukes, and blood flukes.

Rotifers

Rotifers are bilaterally symmetrical, triploblastic animals. They are tiny – most cannot be seen without a microscope. Despite their size (no more than 2mm), they have a complex anatomy. Their nervous system includes a brain and one to five eyes. Rotifers have a complete digestive system with a mouth, stomach, intestine, and anus. The mouth is surrounded by a corona – a crown of cilia that beat to produce a current that directs water and food particles into the mouth.

Rotifers may be sessile or free-living. They are mostly freshwater organisms, but some are marine, or live in wet terrestrial environments. Rotifers that reproduce sexually are dioecious; males and females are separate individuals. However, the bdelloid rotifers are famous for reproducing solely by asexual (mitotic) parthenogenesis. All bdelloid rotifers are female. A third type of rotifers can switch between sexual reproduction and parthenogenesis. There are about 2,200 species of rotifers, although their classification is in flux.

Molluscs

Phylum Mollusca is a large clade of bilaterian, triploblastic animals that inhabit marine, freshwater, and terrestrial ecosystems. Molluscs are characterized by three main body parts: a foot, a visceral mass, and a mantle, which may secrete a shell. Molluscs have a complex nervous system with a brain and two nerve cords (although bivalves are an exception, with three ganglia and three nerve cords). Most have eyes, and the cephalopods have extremely well-developed, camera-like eyes. The digestive system is complete. The mouth is often equipped with a rasp-like tongue, called the radula. They have an open circulatory system (except for cephalopods).

Reproduction varies within molluscs. Gastropods and bivalves may be either dioecious or hermaphroditic. Hermaphrodites may self- or cross-fertilize. Chitons and cephalopods are dioecious, and sexual selection and sexual dimorphism is present in some species. Marine molluscs often have a trochophore larval phase before taking on the characteristics of the adult.

There are around 85,000 described mollusc species. There are five well-characterized extant classes, plus two minor class that may be polyphyletic (and are not listed below).

Mollusc groups

• Class Polyplacophora – chitons
• Class Bivalvia – mussels, oysters, clams, scallops, etc.
• Class Gastropoda – snails, slugs, nudibranchs, limpets, abalone
• Class Cephalopoda – octopus, cuttlefish, squid, nautilus
• Class Scaphopoda – tusk shells

Annelids

Annelids are bilaterally symmetrical, triploblastic worms with a true coelom. Annelids are also often referred to as the segmented worms. Most are indeed segmented; however, a sizeable minority of species show no segmentation. In fact, two groups of unsegmented worms (Echiura and Sipuncula) that were considered to be separate phyla until recently, have now been reclassified as annelids based on molecular phylogeny.[1]  In species that are segmented, most segments are almost exactly the same, and contain a number of specialized organs. Septa separate the body segments. Most annelids have a complete gut. The circulatory system is usually closed. The nervous system usually consists of a brain and two nerve cords that run the length of the body, although many species have fused the two nerve cords into one. Many marine species have eyes, either compound or camera-like.

Reproduction is generally sexual with a variety of life cycles. Earthworms and leeches are hermaphrodites, but many marine annelids are dioecious. Some species are sequential hermaphrodites, meaning they switch sexes at some point during their life.

There are about 22,000 described species of annelids. Most textbooks still describe polychaetes, oligochaetes, and leeches as separate groups. The polychaetes, which are characterized by protrusions extending from each segment (parapodia), are not a monophyletic group and need to be reclassified. Oligochaetes (including earthworms) and leeches belong to the same class, Clitellata, which also includes the echiurans (spoon worms).