animal are so arranged as to leave the stomach consider- 
able freedom of movement. 
The Arenicolide agree with the Scalibregmide in the 
points named above, in the general shape of the body, the 
sub-division of the segments into annuli, the sculpturing 
of the skin, the small lobed prostomium, and the presence 
(in Sealibregma and Humenia) of gills of a similar type. 
The brain and non-ganglionated nerve cord of the caudate 
Arenicolide is similar to that of the Scalibregmidie. 
There are also points of difference between these two 
families which are of considerable importance. In the 
Scalibregmidie the two rami of the parapodia are practi- 
cally identical, but they are very different in the Areni- 
colide. In the latter the neuropodium bears crotchets 
only, and the notopodium bears capillary sete, while in 
the Scalibregmidie both rami bear two kinds of sete, 
capillary and fureate, the latter being characteristic of the 
family. In some of the Sealibregmidee the parapodia 
form laminate appendages bearing dorsal and ventral 
eri, which are not found in Arenzcola (cirrl are oceasion- 
ally seen in the posterior region of American specimens ot 
A. cristata). The gills of Scalibregma and Humenia are 
confined to the first five segments, on which they never 
occur in Arenicola. The heart in the Scalibregmidee is 
a median structure, while in Arenccola there is a pair of 
hearts. The nephridia of the Scalibregmide are minute, 
but numerous, the simple microscopic funnel leading into 
a slender U-shaped tube; the nephridia of Arenzcola are 
few in number and wide and sac-like, each having a large 
funnel fringed with ciliated vascular processes. Several 
of the Scalibregmidie bear complex segmental lateral 
sense-organs which are not found in Arenccola. 
The Arenicolide have a few characters in common 
with the Ophelide, but the accounts of the latter family 
