INTRODUCTION. 



A single specimen of a species of Dolabella was collected at Easter Island 

 by the Tropical Pacific Expedition on 21 December, 1899. An excellent water- 

 color sketch of the animal while living was made by Mr. Agassiz, and it was 

 preserved in alcohol, being later placed in my hands for further study. In 

 view of the notable lack of detailed information upon the anatomy of this genus, 

 I deem the opportunity a valuable one for such a study, and the present paper 

 is offered as a contribution to that end. In addition to the single specimen from 

 Easter Island some use has been made of other specimens of a different species, 

 Dolabella hasseltii (Ferussac) , collected at Samoa and given to me by my friend, 

 Sir Charles EUot, Vice Chancellor of the University of Hong Kong, China. 



SYSTEMATIC CHARACTERS OF THE OPISTHOBRANCHIATA. 



Marine Euthyneura with aquatic respiration; the ventricle of the heart 

 is generally anterior, and the palUal cavity, when present, is widely open. There 

 is a marked tendency to a reduction of the shell, which may become internal 

 or disappear. In the naked forms spicules are sometimes developed. 



Tectibranchiata. 



Hermaphroditic opisthobranchiate Mollusca provided in the adult state 

 with a mantle and shell, with certain exceptions; with one branchial plume 

 and osphradium, with certain exceptions. 



Aplysoidea. 



Shell much reduced, more or less internal, or lost altogether in the adult 

 state. Head with two pairs of tentacles. Margins of the parapodia separate 

 from the ventral surface of the foot, and generally modified into natatory lobes, 



