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- 
¢ olor 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 
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 (Férussac), collected at Samoa and given to me by my friend, 
Sir Charles Eliot, Vice Chancellor of the University of Hong Kong, China. 
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“SYSTEMATIC CHARACTERS OF THE OPISTHOBRANCHIATA. 
Marine Euthyneura with aquatic respiration; the ventricle of the heart 
is ge nerally anterior, and the pallial 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 
Head with two pairs of tentacles. Margins of the parapodia separate 
, ‘om the ventral surface of the foot, and generally modified into natatory lobes. 
