METAZOA VERMES. 295 



with an infinite number of brush-like setae attached on 

 either side and enclosing laterally a double row of deli- 

 cate leaf-like parts. In reality, the long double rod is the 

 ventral side of the worm, and the leaf-like parts are scales 

 or peculiar modifications of the dorsal surface. In this 

 case, gills exist with the scales. The head is differenti- 

 ated from the body. Its anterior portion including the 

 slit-like mouth is dark brown in color, indicating that it 

 performs hard work. 



Psammolyce arenosa (No. 733) covers the upper part of 

 its body with coarse sand, so that at first sight there 

 appears to be a protecting tube. The under side, how- 

 ever, is not covered in this way so that the median groove 

 of the body is distinct and, in lesser degree, the sutures 

 between the segments. Clusters of setae extend down 

 both sides of the body and are useful in locomotion. 



The species of Nereis are typical members of the group 

 of shallow-water worms. The larva of an undetermined 

 species is seen in PI. 734, fig. i. At this early stage it 

 is transparent and consists of a few distinct segments. 

 The head (figs, i, 2) is provided with five pairs of 

 appendages and three pairs of eyes. The large, toothed 

 mandibles (figs, i, 2) are well developed, and even in 

 this early period they can be seen distinctly through the 

 body wall. The setae are prominent, but the parapodia 

 have not yet taken on the form peculiar to the full grown 

 animal. Although the adult Nereis virens Sars (No. 735 ; 

 PI. 736, figs. 1-4), makes a loose flexible tube for itself 1 

 by fasteninig grains of sand together with a secretion of 

 its body, it never attaches itself, but is extremely active, 

 burrowing in sand during the day and often swimming at 

 night. The body is made up of numerous segments of 

 which the forward ones are differentiated into a head (PI. 

 736, fig. i). This part in this species bears six pairs of 

 tentacles and two pairs of eyes (fig. i). The mouth con- 



1 Trans. Conn. Acad. Arts and Sci., Ill, 1878, p. 265. 



