100 MORPHOLOGY OF INVERTEBRATE TYPES 



divided by septa or transverse partitions into a series of cham- 

 bers each corresponding to one segment and connected with the 

 others by openings below the intestine. The Ccelome is lined 

 with peritoneal epithelium. 



Digestive system. The mouth leads into a pharynx which 

 is supplied with protractor and retractor muscles and may be pro- 

 truded in form of a proboscis. On the inner surface of the 

 pharynx, i. e., the one which becomes the outside surface of the 

 proboscis when the latter is protruded, are minute chitinous 

 denticles and two powerful pharyngeal jaws with a serrated edge. 

 The pharynx leads into a short oesophagus. Into the latter open 

 two digestive glands often called salivary glands. The oesophagus 

 leads directly into the intestine into which it projects. The 

 intestine is a straight tube running through the entire 

 length of the worm and terminating on the last segment in 

 an anus. 



The respiratory organs have been already mentioned. They 

 are the ligulce of the parapodia. 



The circulatory organs consist of two longitudinal vessels of 

 which one is dorsal and the other ventral. Both are contractile. 

 The dorsal vessel situated above the intestine in the middle line 

 of the body between the two dorsal longitudinal muscular bands 

 propels the blood towards the head. The ventral vessel situated 

 below the intestine propels the blood toward the posterior end of 

 the body. Both vessels are connected in each segment by a 

 right and a left transverse vessel, forming a ring around the in- 

 testine. Each ring gives off two dorsal and two ventral branchial 

 vessels to the parapodia. The dorsal vessels form a network in 

 the ligula of the upper ramus, the ventral vessels in that of the 

 lower ramus. 



The excretory system consists of paired nephridia, one pair 

 for each segment except head and anal segment. Each ne- 

 phridium is a coiled tube opening into the body cavity by means 

 of a ciliated funnel and to the outside by means of a nephridiopore 

 situated on the ventral surface at the base of the parapodium. 



