Parapod.ia hardly projecting; palps of prosomium forming branched gills; no pharynx or eversible buccal region; no septa in thorax, septa in abdomen regularly disposed.
Behind this point there is a muscular pharynx or gizzard, which communicates with the wide intestinal tract.
The mouth is minute and the pharynx is always suctorial, never gizzard-like.
On the floor of the pharynx or buccal mass is a rudimentary radula, which in many species consists of a single large tooth, bearing two small teeth or a row of teeth.
The mouth is terminal or subterminal; there is a weak sucking pharynx situated behind the brain, and a long intestine lying along the medio-ventral body-cavity; it ends in a cloaca which receives the vasa deferentia in the male.
The pharynx projects freely into the atrium; it is surrounded at the sides and below by the continuous atrial cavity, but dorsally it is held in position in two ways.
An anterior median diverticulum of the pharynx (fig.
The gill-clefts open directly from the cavity of the pharynx into that of the atrium, and so give egress to the respiratory current which enters the mouth with the food (fig.
In both, the alimentary canal takes a straight course from the pharynx (which bends under it downwards and backwards towards the mouth in Limulus) to the anus, and is a simple, narrow, cylindrical tube (fig.
The mouth opens through a narrow pharynx (p) into a chamber which is (as in Crustacea) at once crop and gizzard, the mastax (ma), whose thickenings are imbedded in the posteroventral wall.