Commonly the nephridia are strictly paired a single pair to each segment, while the branches of the blood vascular system are similarly metameric. The alimentary canal is nearly always a straight tube running from the mouth, which is surrounded by the first segment of the body and overhung by the prostomium, to the anus,which is then either surrounded by the last segment of the body or opens dorsally a little way in front of this.
The head of the insect contains a muscular pharynx by means of which the blood from the wound inflicted by the proboscis (labium) is pumped into the alimentary canal and the so-called sucking-stomach.
The size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which measures from 10 to 65 cms. The adults live in great numbers in the alimentary canal of some vertebrate, usually fish, the larvae are as a rule encysted in the body cavity of some invertebrate, most often an insect or crustacean, more rarely a small fish.
The alimentary canal is simple and a gizzard or oesophageal diverticula rarely developed.
In the parasitic Rhizocephala and in a few Copepoda (Monstrillidae) the alimentary canal is absent or vestigial throughout life.
In spite of these difficulties it must be conceded that the dorsal flexure of the alimentary canal of the Pterobranchia is very Phoronis-like.
It is certain that the absorbed juices do not occupy the alimentary canal alone, but pass also into its caecal off-sets which are the ducts of the gastric glands (see fig.
There are no alimentary canal or specialized organs for circulation or for respiration.
The alimentary canal is straight, the anus terminal, though in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it.
They are usually found in the alimentary canal or its appendages but occasionally work their way into the serous cavities, nervous system and blood vessels.