PHYLUM ANNULATA 



473 



In the Oligochreta branchiae 

 are rarely present ; but in certain 

 of the Naiidomorpha there are 

 metainerically 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 peri- 

 toneum. The cuticle, composed 

 of a chitinoid material, usually 

 presents two sj 7 stems 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 con- 

 sists of a single row of cells, in 

 some cases, with smaller cells 

 of replacement intercalated be- 

 tween 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 some- 

 times flattened, sometimes cubi- 

 cal or polyhedral, but more 

 usually more or less vertically 

 elongated. Cilia occur on the 

 surface in certain parts in many 

 Chsetopoda. Among the ordin- 

 ary cells of the epidermis there 

 are usually numerous unicellu- 

 lar glands often containing rod- 

 like bodies. In the tubicolous 

 forms these unicellular glands 

 are active in secreting the ma- 

 terial for the construction of 

 the tube. In addition, the epi- 

 dermis frequently contains sen- 

 sory 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 



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