ANNELIDA
No single feature distinguishes Annelids from other invertebrate phyla, but they have a distinctive
combination of features. Their bodies are long, with segments that are divided externally by shallow
ring-like constrictions called annuli and internally by septa.Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen and does not molt. The basic annelid form consists of multiple segments, each of which has the same sets of
organs and, in most polychaetes, a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly-defined or absent in some, and Echiura and Sipuncula show no
obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles to act as hearts.
combination of features. Their bodies are long, with segments that are divided externally by shallow
ring-like constrictions called annuli and internally by septa.Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen and does not molt. The basic annelid form consists of multiple segments, each of which has the same sets of
organs and, in most polychaetes, a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly-defined or absent in some, and Echiura and Sipuncula show no
obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles to act as hearts.