Paekeb. — Metridiuiu marginatum. 



PLATE II. 



Fig. 4. Lateral view of specimen D, with two complete oral disks, collected by the 

 writer and S. F. Williams at Beverly Bridge, Mass. The arrangement 

 of the internal parts of this specimen as seen in transverse section is 

 shown in Figure 9 (Plate III.). Female. Natural size. 



Fig. 5. Oral view of the same specimen as that shown in Figure 4. Slightly 

 enlarged. 



Fig. 6. Transverse section of specimen A, with one oral disk and two mouths. 

 The section is taken at a level at which the oesophagus is single. 

 Orally from this the oesophagus becomes double, each tube leading to 

 a mouth opening. Sex not determined. Collected by B. H. Van Vleck 

 at Cape Ann, Mass. X2. 



Fig. 7. Transverse section of specimen B, with one oral disk and two mouths. The 

 cavities of the two oesophageal tubes were entirely distinct except near 

 the extreme aboral end, where the two tubes united to form a single 

 oesophagus. The oesophageal tubes are connected with each other by 

 a pair of membranes which resemble mesenteries without differen- 

 tiated muscle bands. These membranes extend from the region in 

 which the two oesophageal tubes separate almost to the oral disk. The 

 cavity between them opens into the gastrovascular cavity orally, since 

 both membranes are deficient in that region. Mesentery 1, though 

 complete orally, is incomplete through more than half of its pedal 

 length. Collected by the writer at Wood's Hole, Mass. Female. X3. 



