ALCIOPIDAE. 129 



Alciopidae. 



The alciopids are exclusively pelagic forms, in which the body is for the 

 most part unpigmented and of crystalline transparency. A few forms are 

 brown, or, more commonly, violet in color. They are ordinarily small, or but 

 moderate in size, with the body cylindrical. 



The prostomium bears always four paired tentacles and a median unpaired 

 tentacle. There are no palpi. A highly characteristic feature in the family 

 is in the exceptional development of the two eyes, which are normally very 

 large, protruding conspicuously on each side of the head. 



Nuchal organs rudimentary, or wholly lacking. 



A number of somites behind the prostomium are obviously different, as 

 in the character of their parapodia, from the succeeding ones, the first normal 

 parapodia appearing at a considerable distance from the prostomium. 



The parapodia are uniramous with notocirri and neurocirri foliaceous and 

 containing well-developed mucous glands. The parapodia may or may not 

 have at their distal ends cirriform prolongations. 



The setae may be all simple, all composite, or both composite and simple 

 types may be present. Simple setae of a stouter, crochet-type are often present. 



The nephridia are as in the phyllodocids and other members of the Phyllo- 

 docoidea. 



Proboscis either unarmed, or with small denticles. 



An interesting feature in this family is that some and possibly all of the 

 species, while free as adults, in larval stages live symbiotically or parasitically 

 on or in other pelagic animals. The genus Alciopina of Claparede and Panceri 

 (Mem. Soc. Ital. sci. nat. 1867, 3, no. 4, p. 8, fig. 2-15), according to Greeff, (Nova 

 acta Acad. Caesareae Leop. -Carol., 1876, 39, p. 45) was apparently founded on 

 such a larval form found in the alimentary tract of Cydippe densa Forskal. 



The close affinity of this family with the Phyllodocidae sens. str. is evident, 

 not only in external morphology, but in detailed internal structure as well. 



The alciopids have been found in all the great oceans, but the great majority 

 have been secured in the Atlantic and in the Mediterranean Sea. Apstein 

 (Ergebn. Atlant. Ocean * * * Plankton-exped., 1900, 2, H. b., p. 22) lists eighteen 

 species as known from the Atlantic, eleven from the Mediterranean, six each 

 from the Pacific and Indian Oceans, and one from the Antarctic. Izuka (Journ. 

 Coll. sci. Imper. univ. Tokyo, 1914, 36, p. 2-9) mentions four additional species 

 from off Japan, making a total from the Pacific of ten species previously known. 



