STRUCTURE 



491 



layer and an oblique layer which runs across the body-cavity 

 from the dorso-lateral surface to the mid-ventral line, a primitive 

 arrangement which recalls the similar division of the body-cavity 

 into three chambers in Pcripatus and in many Chaetopods. 

 Besides these there are certain muscles which move the hooks 

 and other structures. 



The mouth opens into a pharynx which runs upwards and 

 then backwards to open into the oesophagus (Fig. 257). Certain 

 muscles attached to these parts enlarge their cavities, and thus 

 give rise to a sucking action by whose force the blood of the 

 host is taken into the alimentary canal. The oesophagus opens 

 by a funnel-shaped valve into the capacious stomach or mid-gut, 



FIG. 257. Diagrammatic representation of the alimentary, secretory, nervous, and repro- 

 ductive systems of a male Porocephalus teretiusculus, seen from the side. The 

 nerves are. represented by solid black lines. (From W. Baldwin Spencer.) 



1, Head-gland; 2, testis ; 3, hook-gland; 4, hind-gut ; 5, mid-gut; 6, ejaculatory 

 duct ; 7, vesicula seminalis ; 8, vas deferens ; 9, dilator-rod sac ; 1ft, cirrus-bulb ; 

 11, cirrus-sac ; 12, fore-gut ; 13, oral papillae. 



which stretches through the body to end in a short rectum or 

 hind-gut. The anus is terminal. 



There appears to be no trace of circulatory or respiratory 

 organs, whilst the function usually exercised by the nephridia 

 or Malpighian tubules or by coxal glands, of removing waste 

 nitrogenous matter, seems, according to Spencer, to be transferred 

 to the skin-glands. 



The nervous system is aggregated into a large ventral ganglion 

 which lies behind the oesophagus. It gives off a narrow band 

 devoid of ganglion -cells, which encircles that tube. It also 

 gives off eight nerves supplying various parts, and is continued 

 backward as a ninth pair of prolongations which, running along 

 the ventral surface, reach almost to the end of the body (Fig. 257). 

 The only sense-organs known are certain paired papillae on the 

 head, which is the portion that most closely comes in contact 

 with the tissues of the host. 



