2 JEAN DAWSON. 



(Fig. I, b\ From the mouth cavity it passes through a constric- 

 tion into the larger so-called pharyngeal cavity (</). From the 

 pharyngeal cavity food enters the alimentary canal proper by 

 way of the slender oesophagus (/>) while water may conceivably 

 enter the much larger water tube (s). The latter lies ventral to the 

 oesophagus, extends caudad to the pericardial cavity, and there 

 ends blind. Should water enter it from the pharynx it could 

 then pour through the seven openings on each side into the 

 gill sacs, from which it could reach the exterior by the seven 

 external branchial openings on each side of the body (Fig. 



5, /') 



The opening from the pharynx into the water tube is guarded 



by a pair of velar valves (Fig. I,/; Fig. 13, /), while valves guard 

 also the external branchial openings (Fig. 13, c, &). Muscles 

 control the opening between mouth cavity and pharynx and 

 between pharynx and oesophagus. In order to understand the 

 mechanism of these parts, each must be considered in greater 

 detail. 



A. Oral Funnel, Month Caiity and Tongue. The oral fun- 

 nel opens upon the ventral side of the anterior part of the 

 head. In the specimens examined the average diameter of its 

 external opening is 3.5 cm., while its dorsal opening, where it 

 becomes continuous with the mouth cavity, has a diameter of 

 i cm. Around the smaller opening of the funnel, on the boun- 

 dary between it and the mouth cavity, is a supporting ring of 

 cartilage, called the annular cartilage (Fig. I, /). Attached to 

 this cartilage and forming the muscular walls of the funnel is 

 a three-layered muscle, the annularis (Fig I, //). Furbringer 

 (1875) reports that the outer and middle layers of the annularis 

 serve to attach the animal by the oral funnel while the inner 

 layer causes the walls of the funnel to expand and contract, 

 and this, together with the simultaneous closing of the mouth 

 opening (opening between the oral funnel and mouth cavity), 

 gives the animal power to suck blood or tissue which it has 

 succeeded in rasping from its prey. The walls of the fun- 

 nel become thin abruptly at their external edge, and there 

 bear a thick fringe of rather short tentacles (Figs. I, z; Fig. 

 2, /). 



