314 THE EARTHWORM AND OTHER ANNULATA 



with the Sedentaria, or fonns that " sit down." The latter group 

 included many polychaetes that build tubes and thus become 

 more or less permanently fixed in one locality. Many sedentary 

 polychaetes, like the tube-building species, Hydroides, and Cirrahi- 

 lus, have remarkably developed feeding and respiratory organs at 

 the anterior end of the body. Cistenides forms a conical tube of 

 sand grains cemented together. Choetopterus produces a leathery 

 U-shaped tube through which it causes a circulation of the water, 

 thus bringing food and the oxygen within reach. 



The Annulate Plan ot Body. — In the body plan of the more 

 representative types of the Phylum Annulata (cf. Figs. 137 D and 

 140), an outer tube, the body wall, surrounds a coelomic cavity and 

 an inner tube, the digestive tract, which extends from end to end of 

 the animal. The ccelome is divided by septa between the seg- 

 ments or metameres. The paired excretory organs are seg- 

 mentally arranged so that the inner opening is in one segment and 

 the outer opening in the one just posterior. There are dorsal and 

 ventral blood vessels, connected laterally in each segment. The 

 nervous system consists of a double chain of ventral ganglia with 

 their connectives, together with the circum-pharyngeal connec- 

 tives, and the paired cerebral ganglia, which form the dorsally 

 placed brain. In the simpler types the sexes are separate, there are 

 reproductive organs in each segment, and the nephridia serve as 

 ducts for the spermatozoa and ova. Certain segments may be 

 specialized externally or internally, as is the case toward the 

 anterior end of the earthworm and Nereis; but, in the general 

 scheme of the body, every metamere is the equivalent of every 

 other one, since it contains similar parts of each system of organs. 

 Metamerism is, therefore, one of the distinctive features of the 

 annulate body. Only two other phyla of the Animal Kingdom, the 

 Arthropoda and the Chordata, exhibit this type of organization, the 

 simplest form of which is shown by those annulates in which large 

 portions of the body are merely repetitions of similar segments. 

 Metamerism, the coelome, and a triploblastic structure, together 

 with the nephridia and the arrangement of the nervous system, are 

 thus the most important characteristics of the body plan in the 

 Phylum Annulata. 



