PENTASTOMIDA. 861 



alimentary canal to near the hinder end of the body. Anteriorly 

 the right and left glands fuse to form the head-gland which fills 

 up a good deal of the space within the head. From the head- 

 gland three ducts arise on each side, one passes to each of the 

 large papillae and one to the base of each hook ; it has been sug- 

 gested that the glands secrete an irritating fluid which stimulates 

 the flow T of blood and prevents a coagulum being formed. 



Below the epidermis lies a layer of circular muscles and be- 

 neath them a longitudinal layer. As in Polygordius, Peripatus 

 and other primitive animals, a sheet of muscle fibres runs from 

 each side of the body towards the ventral median line thus 

 dividing the body cavity into three chambers. Other muscles 

 move the hooks, and there are several which by contracting 

 enlarge the lumen of the pharynx which thus acts as a sucking- 

 pump. The muscles are striated. 



The mouth is borne on a prominence ; it leads into a pharynx 

 which passes backwards and upwards and soon opens into an 

 oesophagus. This passes upwards and projects into the lumen 

 of the stomach, thus forming a valve which prevents the regur- 

 gitation of the fluid food. The stomach or mid-gut is large 

 and stretches back to near the hind end of the body ; here it 

 narrows and passes into a short rectum which opens by a terminal 

 anus. The pharynx and oesophagus and the rectum are lined 

 w r ith chitin and represent the stomodaeum and protodaeum 

 respectively. 



The nervous system consists of a large ventral ganglion which 

 gives off two cords devoid of ganglionic cells which meet and 

 fuse above the oesophagus. From each side eight nerves arise 

 which supply the neighbouring structures, and behind, a ninth 

 pair, the largest of all, runs backward along the ventral sur- 

 face of the animal to the posterior end. The only sense organs 

 are certain paired sense papillae situated on the head. There 

 are no specialized respiratory or circulatory organs and no organs 

 comparable with nephridia, coxal glands or malpighian tubules. 



The sexes are separated in the Pentastomida. The males 

 are smaller than the females and are said to be the more numerous. 

 The testis is a cylindrical sac closed behind, lying dorsally above 

 the stomach. Anteriorly the sac splits and passes into two 

 lateral ducts, the vesiculae seminales, which encircle the alimen- 

 tary canal and pierce through the tissue of the hook-glands. The 



