Annelids. Worms are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, earthworms have dual nerve cords running along the length of the body and merging at the tail and the mouth.

First, we would include here those beings that do not have a nervous system, such as Porifera (the phylum that includes sponges), and those who do have a nervous system which is not centralized, such as echinoderms and cnidarians. Non-sentient animals would then include sponges, corals, anemones, and hydras.

The flatworms were the first invertebrates to exhibit bilateral symmetry and also the first to develop a central nervous system with a brain.

– Sponges reproduce by both asexual and sexual means. Most poriferans that reproduce by sexual means are hermaphroditic and produce eggs and sperm at different times.

Cnidarians contain specialized cells known as cnidocytes (“stinging cells”) containing organelles called nematocysts (stingers). These cells are present around the mouth and tentacles, and serve to immobilize prey with toxins contained within the cells.

A cnidocyte is an explosive cell containing one giant secretory organelle or cnidae that defines the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). Cnidae are used for prey capture and defense from predators.

Nematocysts are the means by which coelenterates capture prey and defend against predation. The 25 or more known types of nematocysts can be divided into to four functional categories: those that pierce, ensnare, or adhere to prey, and those that adhere to the substrate.

Cnidarians have cnidocytes, or stinging cells, which penetrate and inject toxins into their prey, whereas ctenophore tentacles have distinctly different colloblasts, or sticky cells, that are used to entangle prey until they can bring them to their mouth and consume them.

Like cnidarians, ctenophores also exhibit extra and intracellular digestion. Reproduction is sexual with indirect development. Cnidoblasts are absent hence these are called acnidarians.

Comb jellies aren’t harmful to humans, but they wreak havoc on the local ecosystem. In the Adriatic Sea, they don’t have any predators yet. ctenophore into the region, which just goes to show that even in the animal kingdom, sometimes the enemy of an enemy is a friend.

Ctenophores are free-swimming, transparent, jelly-like, soft-bodied, marine animals having biradial symmetry, comb-like ciliary plates for locomotion, the lasso cells but nematocytes are wanting. They are also known as sea walnuts or comb jellies.

In Ctenophores: 1. Sexes are separate. 2. Reproduction takes place only by sexual means.

Pleurobrachia — The Sea gooseberry: The tentacles bear special adhesive cells, the lasso cells or colloblasts which help in food capture. The broad end called aboral pole, contains the anal canals, anal pores and a sense organ, the statocyst. The animal is hermaphrodite.