78 ESSENTIALS OF ZOOLOGY 



The dorsal pores, absent in aquatic types, may be seen by 

 drying the worm with a cloth and stretching it over a finger. 

 This serves to press out some of the fluid of the body cavity. 

 Dorsal pores begin mid-dorsally in the interval between the 

 tenth and eleventh segments, and occur between all the remain- 

 nig segments to the groove separating the penultimate from 

 the last segment. Each dorsal pore leads into the posterior 

 of the two segments in each case. The nephridial openings, 

 or nephridiopores, begin on the fifth segment, and a pair is 

 found on every segment from that one to the penultimate 

 segment. The openings are difficult to distinguish. They 

 lie one on each side, usually just behind and a little above the 

 lower of the two sets of chaetae. 



A thin cuticle is secreted by the ectoderm, and it forms a 

 smooth delicate covering to the skin. It becomes plain if 

 dissection is performed in water, and may then be readily 

 stripped off and examined. It is a hardened exudate of the 

 ectoderm, and the chaetae are of the same nature and are 

 developed in special pockets of the skin, from which they may 

 be protruded and into which they may be withdrawn by 

 special muscles of the skin. 



Sections will show that the ectoderm consists of a series of 

 columnar cells forming a single layer, thicker in the middle of 

 the segment and thinner in the grooves. The ectoderm is 

 based on a basement membrane. Glandular cells are inter- 

 spersed among the columnar cells, and their secretion escapes 

 by pores in the cuticle. In the segments involved in the 

 clitellum the ectodermal cells become largely increased in 

 number and glandular. 



INTERNAL MORPHOLOGY. Alimentary Canal. Like the 

 ectoderm with which it is continuous at either end of the body, 

 the epithelium of the alimentary canal is formed of a single 

 layer of columnar cells. The muscles of the two layers are 

 similar and continuous. Underneath the ectoderm are circular 

 and then longitudinal muscles, the latter continuous and the 

 former interrupted at the grooves. Around the alimentary 

 canal the muscle layers are an inner circular layer and an outer 

 longitudinal layer (fig. 38). If the worm be dissected from 

 the dorsal side it will be found that the alimentary canal 



