x PHYLUM ANNULATA 435 



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

 chitinoid material, usually presents two systems of fine lines inter- 

 secting 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, with, in some cases, 

 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 

 epidermis there are usually numerous unicellular glands often 

 containing rod-like bodies. In the tubicolous forms these unicel- 

 lular glands are active in secreting the material for the construc- 

 tion of the tube. In addition the epidermis frequently contains 

 sensory cells, which are in many cases contained in certain special 

 elevations or sensory papilla?. 



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, interrupted opposite the intervals 

 between the segments. The longitudinal layer is disposed in four 

 bands in the Polycha?ta, two dorso-lateral and two ventro-lateral. 

 In the Oligochseta it is divided by the setigerous sacs which pass 

 through it. 



The peritoneum or ccelomic epithelium consists of a single layer of 

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

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

 bringing about an active circulation of the fluid in the ccelome, 

 the ccelomic o\- peri visceral fluid. 



The body-cavity or ccclotnc, 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 Chastopoda by a series of 

 transverse septa or mesenteries into a series of chambers corre- 

 sponding 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 

 invagination of the ectoderm, and therefore correspond to a stomo- 

 dceum. The muscular pharynx is absent in some of the tubicolous 



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