POLYCHAETOUS ANNELIDS 

 Part VII. Capitellidae 



(Plates 43-58, 1 Chart) 

 By Olga Hartman 



This constitutes one of the oldest known groups of polychaetous anne- 

 lids; it was already known in the late seventeen hundreds through the 

 species Capitella capitata (Fabricius). The affinities of the group with 

 other polychaetous annelids, however, were not clarified until detailed 

 anatomical studies of organ systems had verified and proved their true 

 relations. They had been variously considered as oligochaetes, repre- 

 sentatives of Maldanidae, Arenicolidae, and Serpuliformia even as late as 

 1865 (see Eisig, 1887, pp. 1-10, for detailed history). 



The family name was first erected as the Capitellacea by Grube 

 (1862) when Notomastus, Dasybranchus, and Capitella were correctly 

 grouped together. A year later the family name, Halelminthea Carus, 

 was proposed for Capitella, but this author considered it in the Oligo- 

 chaeta together with an opheliid, Polyophthalmus; the name Halelmin- 

 thea now has only historical interest. Today the various genera and 

 species of Capitellidae (see below) are indisputably considered distinct, 

 forming a closely related family; their affinities are with other sedentary 

 chaetopods. 



Externally and grossly considered, many capitellids resemble ter- 

 restrial oligochaetes because of their plain, unadorned exterior. In 

 details, however, the body is seen to consist of a distinct prostomial lobe, 

 a thorax with a limited number, and an abdomen with a variable number 

 of segments. The thorax and abdomen, though distinct from each other, 

 are sometimes indivisible in external view or are separate from each other 

 by transitional segments (pi. 58, fig. 1). The prostomium is a plain, 

 short, rounded (pi. 55, fig. 1) or elongated (pi. 47, fig. 4) lobe, without 

 eyes (pi. 50, fig. 4) or with a pair of pigmented areas (pi. 47, fig. 4) at 

 the sides. A pair of eversible nuchal organs or ciliated mounds, homo- 

 logous with the lateral organs farther back, is more or less conspicuous at 

 the posterior margin of the prostomium, or may be reduced nearly to dis- 



[391] 



