XX SHRUG RU RIE 4QI 
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 Peripatus 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, 
N 
AA 
NSS 
7 
Fic. 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 ; 10, 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. 
