PLATE XXXVII 



a, proboscis; b, ganglion; cc, collar cavity; ch, cavity of heart; cp, collar 

 pore; d, notochord; dv, dorsal blood vessel; eg, external gill-slit; gr, 

 roof of gill slit; h, pharynx;/, pericardium ; /c, pericardial cavity; p,pc, 

 proboscis cavity ; pt, problematic tissue ; pv^ and pv^ , pharyngeal blood 

 vessels; vv, ventral blood vessel. 



Fig. I. C. densus, sagittal section through collar region. 



Fig. 2. C. densus, sagittal section through collar region. 



Fig. 3. C. densus, transverse section through the heart and pericardium. 



Fig. 4. C. densus, transverse section of heart and pericardium. 



Fig. 5. C. nigrescens, transverse section passing through heart and 

 pericardium. 



