MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 155 



flanges of the lower jaws extend far back, forms two deep pockets. The buccal membrane fits 

 closely around the jaws and reduces the space between itself and them to a minimum. 



The inner fold of the buccal membrane is somewhat thickened dorsally, and here it bears, a 

 few millimeters back of the papilla?, two parallel transverse folds extending across the middle 

 line. The folds are very thin, about IS millimeters in length by 2 millimeters to 3 millimeters 

 in height, and are close together. 



When we slit open the cephalic sheath and buccal membrane dorsally we expose the base of 

 the buccal mass, the oesophagus, and cephalic commissure crossing the (esophagus. (Figs. 7 and 28.) 

 The hsemocoelic space existing between the buccal membrane and the base of the buccal mass has 

 been mentioned. This is in communication with a labyrinth of similar spaces extending between 

 the muscles and membranes of the buccal mass and cephalic sheath. These are further in com- 

 munication with the main part of the hsemocoel which surrounds the (esophagus and liver. Many 

 of the membranes and small muscles found around the base of the buccal mass are very inconstant 

 in their occurrence and extent. 



Strong muscles attach the buccal mass to the cephalic cartilage and body wall and govern its 

 motions as a whole. Six muscles, arranged in pairs, seem to be retractors of the buccal mass. 

 Four of these are dorsal and two are ventral. It seems convenient to distinguish the two pairs 

 of dorsal muscles as the. dorsal retractors and the dorso-lateral retractors. The dorsal retractors 

 arise near the center of the cartilage and run inward and forward upon the buccal mass to their 

 insertions dorsally between the flanges of the upper jaw. (Fig. 28, DR.) The dorso-lateral 

 retractors have their origins immediately beneath those of the dorsal retractors (in contact with 

 them, in fact). They run under the dorsal retractors to the buccal mass, where they spread out 

 fanwise over the sides of the buccal mass and are inserted at the edge of the external flange of 

 the lower jaw. (Fig. 28, DLR.) The development of the dorso-lateral retractors seems to be 

 more or less variable; when best developed they are fully as distinct and as strong muscles as the 

 dorsal retractors. The cerebral ganglia lie just back of the buccal mass and directly under these 

 muscles (Fig. 28, CG); from them a number of fine nerves pass upward between the retractors 

 to the dorsal parts of the buccal mass and the edge of the buccal membrane. (Fig. 28, N.) 



The ventral buccal retractors have their origins upon the body of the cartilage and run 

 forward and outward upon the under surface of the buccal mass to be inserted along the lower 

 edge of the external flange of the lower jaw. These muscles are sometimes divided into more or 

 less distinct parts. The retractors of Van der Hoeven's organ, which are partly attached to 

 the base of the buccal mass, may pass above the ventral retractors or between the muscle bundles 

 in case the muscles divide. (Fig. 29.) 



A narrow band of muscle fibres extends along the median line of the inner surface of the 

 outer fold of the buccal membrane into the body wall, the median muscle of the buccal membrane. 

 From either side of the median muscle a broad bandlike muscle passes outward and downward to 

 the ventral side of the buccal mass, where its fibres mingle with those of the ventral retractors 

 and are attached to the edge of the lower jaw. (Figs. 28 and 33, LM.) These muscles are 

 evidently levators of the buccal mass, as they hold it suspended in a sort of sling: It Mill be 

 noticed that the six retractors and the two levators spread out considerably over the surface of 

 the buccal mass and that their fibres commingle to a certain extent. The muscles are connected 

 by a muscular membrane and thus, by the union of the distal portions of the muscles and this 

 membrane, a complete covering is formed to the posterior part of the buccal mass, extending 

 from the edges of the jaws to the circum-oesophageal nerve ring and firmly united to the tough 

 covering of the nerve ring. (Figs. 28, 33, and 34, MM.) A second, more ventral membrane is 

 stretched between the ventral retractors near their origins, thus forming an apparently closed 

 chamber. (Fig. 33, OM.) Another membrane may extend between the levator muscles and the 

 outer fold of the buccal membrane. Ventrally this membrane may be continued by the edges of 

 Van der Hoeven's organ, as in Fig. 20, though it is rarely anything like as complete as it was 

 in this specimen. 



Upon laving open the muscular membrane, which has been described as covering the buccal 

 mass back of the jaws, a very intricate system of intrinsic buccal muscles is exposed. Observing 



