Phylum Annelida Lecture Notes

Introduction to Annelids

As animals evolved, they developed different tissue layers:
- Ectoderm: Outer embryonic layer; forms skin and nervous tissue
- Endoderm: Lines the digestive tract
- Mesoderm: Develops into muscles, bones, circulatory system, and organs
Cnidarians: Have ectoderm and endoderm separated by noncellular mesoglea

Flatworms: Three tissue layers, no body cavity, double-walled sac with a single opening
Roundworms: Pseudocoelom (body cavity between mesoderm and endoderm)
Annelids: True coelom (tube within a tube body cavity)
- Advantages:
  - More room for organs
  - Can function as a circulatory system
  - Efficient digestive system
  - Room for gonads, gametes
  - Acts as a hydrostatic skeleton

Septum: Internal walls; most segments identical but some specialized for functions
Feeding:
- Food enters through mouth, travels through gut, and is digested
- Pharynx: Muscular front end of digestive tube, varying among annelids
Respiration:
- Aquatic annelids use gills
- Terrestrial annelids breathe through skin (must be moist for gas exchange)
Internal Transport:
- Closed circulatory system with blood vessels
- Earthworms have longitudinal vessels and five pairs of hearts
Excretion:
- Solid wastes exit through anus
- Cellular wastes removed by nephridia
Response:
- Brain on top of gut; ventral nerve cord runs length of worm
- Sense organs best developed in marine species
- Earthworms lack specialized sense organs
- Defense: Earthworms grab burrow walls
Movement:
- Two muscle groups: longitudinal (shortens body) and circular (makes body skinnier)
Reproduction:
- Mostly sexual reproduction
- Some species have separate sexes, others are hermaphrodites
- Clitellum secretes mucus for egg and sperm release

Class: Oligochaeta
Earthworms: Long pink worms, inhabit soil, fewer bristles than marine polychaetes