THE ANATOMY OF SOALIBREGMA INFLATIJM. 299 



Tlie dorsal vessel of Scalibregma is dilated just behiud 

 the fourth diapliragm to form the hearty which is therefore 

 a median structure, thus differing entirely from the hearts of 

 Arenicola, which are paired, and not directly connected 

 with the dorsal vessel. The nephridia of Scalibregmidte are 

 minute but numerous, and the simple microscopic funnel 

 leads into a slender U-shaped excretory tube. The nephridia 

 of Arenicola are fewer in number, and are wide sacs, each 

 with a large funnel fringed with ciliated vascular processes. 

 Several of the Scalibregmidse bear complex, segmental, 

 lateral sense organs, which are not found in Arenicola. 



The Scalibregmidge have only a few features in common 

 with the Opheliid^. Besides the points mentioned above as 

 common to the three limnivorous families, they agree in the 

 great development of the muscles of the ventral body-wall 

 (especially in Eumenia and Lipobranchius), the nerve- 

 cord without ganglia, the dorsal heart, and the principal 

 features of the circulatory system. The resemblances may 

 be best seen on comparing the Opheliid Ammo try pane 

 cestroides with Eumenia and Scalibregma. The 

 Opheliidae and Scalibregmidge differ in their nephridia, those 

 of Opheliids being comparatively few and sac-like; in their 

 prostomia, that of Opheliids is a single conical outgrowth; in 

 their pai-apodia and setae. The Scalibregmidae have little in 

 common with any other family of Polychaetes. 



We may therefore say that the Scalibregmidge agree in 

 several respects with the Arenicolidas and Opheliidae, and it 

 is difficult to say that they are more related to one of these 

 than to the other, though, on the whole, there are rather 

 more features in which the Scalibregmidte agree with the 

 ArenicolidiB (e. g. shape, secondary annulatiou, sculpturing 

 of skin, character of the gills when present, prostomium, 

 brain, and nerve-cord) than with the Opheliidge. The Scali- 

 bi-egmidee, however, are clearly distinguished from these 

 families by the presence of the peculiar furcate setge in the 

 parapodia, and by their numerous delicate nephridia, among 

 other characters. 



