x PHYLUM ANNULATA 463 



of the Microdrili there are inetamerically arranged simple or 

 branched branchiae, sometimes retractile, on the segments of the 

 posterior region. 



The body-wall consists of a cuticle, an epidermis, muscular 

 layers, and a layer of peritoneum. The cuticle, composed of a 

 chitinoid material, usually presents two systems of fine lines 

 intersecting one another at right angles : it is perforated in many 

 places by the ducts of the unicellular glands of the epidermis. 

 The epidermis consists of a single row of cells, in some cases with 

 smaller cells of replacement intercalated between their inner ends. 

 In shape the cells vary greatly in different families and often in 

 different parts of the body of the same worm, being sometimes 

 flattened, sometimes cubical or polyhedral, but more usually more 

 or less vertically elongated. Cilia occur on the surface in certain 

 parts in many Chsetopoda. Among the ordinary cells of the epi- 

 dermis there are usually numerous unicellular glands often containing 

 rod-like bodies. In the tubicolous forms these unicellular glands 

 are active in secreting the material for the construction of the tube. 

 In addition, the epidermis frequently contains sensory cells, which 

 are in many cases contained in certain special elevations or sensory 

 papillae. 



The muscular part of the body-wall consists of two layers, in the 

 outer of which the fibres are disposed circularly, while in the inner 

 their arrangement is longitudinal. The circular layer is continuous, 

 or, more usually, is interrupted opposite the intervals between 

 the segments. The longitudinal layer is disposed in four bands in 

 the Polychseta, two dorso-lateral and two ventro-lateral. In the 

 Oligochseta it is divided by the setigerous sacs which pass through it. 



The peritoneal or ccelomic epithelium consists of a single layer of 

 cells. These are usually non-ciliated, but are ciliated in the 

 Aphroditea, Glycera, and some others, the movement of the cilia 

 bringing about an active circulation of the ccelomic or perivisceral 

 fluid in the ccelome. 



The body-cavity or ccelome, a wide space intervening between 

 the wall of the body on the one hand and that of the enteric canal 

 on the other, is divided in many Chsetopoda by a series of transverse 

 septa into a series of chambers corresponding to the segments. 

 The septa are not complete partitions, there being always apertures 

 of greater or less extent by which the cavities of neighbouring 

 segments communicate. The septa consist of double folds of the 

 peritoneum enclosing muscular fibres. 



The enteric canal is an elongated and nearly always straight 

 tube, running through the entire length of the body from mouth 

 to anus. A number of different parts are usually distinguishable ; 

 but their disposition varies to a very great extent in the different 

 groups. The buccal cavity, into which the mouth leads, is followed 

 by a muscular pharynx ; these are both formed in the embryo by 



