230 ZOOLOGY 



are also dorso-ventral fibres. Special groups of fibres occur in 

 connection with the setae, the mouth parts, suckers, etc. The 

 fibres are spindle-shaped and unstriate. The dermo-muscular 

 wall bounds a true body cavity in the chsetopods ; but in leeches 

 the ccelom is almost filled with connective tissue. This sug- 

 gests the condition in many of the unsegmented worms. See 

 Figs. loi, 103. 



266. Worms as a rule have no external skeleton other than 

 the cuticula, but in some instances a tubular protective structure 

 is formed by secretion or by cementing together small particles 

 of foreign matter. Because of the absence of hard skeletal 

 parts little is known concerning the worms of past geological 

 ages. 



267. Digestive System and Feeding. — The stomodasum, the 

 mesenteron, and proctodaeum (see §93) are all to be distin- 

 guished in the enteric canal. The mouth is not quite terminal, 



Fig. 104. Transverse section of the intestine of the Earthworm, ly, typhlosole, an infolded 

 longitudinal ridge in the gut in which the gland cells (gl.) are especially aggregated. Other letters 

 as in Pig. X03. 



Questions on the figure. — Of what conceivable gain is the typhlosole? What 

 is it analogous to in the higher types of animals? 



but slightly ventral. The prostomium (or preoral lobe), a mus- 

 cular extension of the oral segment, overarches it. There is 

 typically an enlarged muscular pharynx which is often eversible, 

 followed by a narrow tubular esophagus. Often there is no 

 further differentiation, the remainder of the tube being fairly 

 uniform and called the intestine. Frequently, however, special 



