312 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- ventral 

 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 nmi. The inner surface of the pseudo-peri toneima 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 disc-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. 



Pharyngeal bulb. — The relatively small pharyngeal bulb is nearly cyUndri- 

 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 sUghtly 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 sUghtly yellowish color of the l)ulb 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 ^\ith 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 



