31 4 THE DOLABELLINAE. 
INTERNAL ANATOMY. 
Owing to the lack of material a complete description of the anatomy of 
Dolabella agassizi cannot here be given, the more important systems alone 
having been studied. 
ALIMENTARY TRACT.— The specimen was dissected from below, a mid-ven’ 
longitudinal incision being carried the full length of the foot. The sole of the 
foot is strong and muscular, reaching a thickness of 3.0 mm. in front and thin- — 
ning away behind to 2.0 mm. The inner surface of the pseudo-peritoneum is — 
greenish black in color: 
The retractor muscles of the head-region are very strongly developed in 
the form of a series of broad bands, arranged in the form of an incomplete cone, ) 
being absent on the ventral side alone. They arise in the posterior portion of 
the body-cavity at varying distances from the posterior dise-region, and are 
inserted at the anterior end in the zone between the rhinophores and the anterior . 
tentacles. It is by the powerful contraction of this group of muscles that 
the complete inversion of the head-region is brought about. 5 
Pharyngeal bulb.— The relatively small pharyngeal bulb is nearly eylindri- 
cal in form, 18.5 mm. long by 14.3 mm. broad and about the same in height. 
The dorsal, ventral, and lateral faces of the bulb are slightly flattened. Ante- 
riorly it receives the short and broad mouth-tube, and posteriorly it continues 
into the wide oesophagus, nearly equal in diameter to the bulb itself, a slight 
external ventro-lateral constriction marking the passage of the one into the 
other, the slightly yellowish color of the bulb giving way to the bluish gray tone 
of the oesophagus. 
In the specimen at hand the complete introversion of the head-region, 
extending back beyond the rhinophores, makes the arrangement of the muscles 
of that region not evident at first sight. The strong retractors of the bulb 
arise close to the union of the mouth-tube with the body-wall, and are scarcely 
to be distinguished from the general system of retractors of the whole head- 
region, their insertion being the same, save for a single pair of dorsal retractors, 
which are inserted above the bulb. 
From the ventral face of the pharyngeal bulb a series of short narrow 
flat muscles arise, which pass forward and are inserted into the anterior end of 
the foot. Four of these inferior protractor muscles are found upon either side 
