402 EK. W. MACBRIDE. 
those of Ptychodera. The collar region is moderately 
long, and the skeletal rods reach right through it. 
They are consequently very long (Pl. 30, figs. 9 and 12). The 
inner gill-pores only reach halfway down the side of the 
cesophagus (fig. 14a), but the lower nutritive portion is not 
constricted from the upper branchial. There are no synap- 
ticula in the gill-skeleton, and no liver saccules. 
There is no circular muscular layer in the trunk, nor any 
system of muscles replacing it. The chondroid tissue 
is very feebly developed. The central nervous system has 
no dorsal roots. 
B. Kupfferi (fig. 7) is the North Sea species. Its proboscis 
has the normal length. It has two proboscis pores 
opening into the right and left dorsal pockets of 
the proboscis celom. (In Ptychodera minuta this 
condition occurs as a variation; we also find one median pore 
opening into both pockets.) The head of the notochord is 
short; the neck has two lateral wings, giving rise to corre- 
sponding projections of the skeleton (Pl. 30, fig.9 W). In full- 
grown animals the hinder part of the neck is broken 
up by invading portions of the skeleton. There are 
no peripharyngeal spaces, but there is a layer of circular 
muscles suspended in the midst of the collar cavity 
by the radiating muscles. There are, as in Glandiceps, additional 
dorsal gonads, and the eggs are large and yolky. There isa 
continuous blood-sinus round the whole gut behind the collar. 
B. canadensis has a short round proboscis, in which the 
cavity is almost obsolete. The longitudinal muscles are 
represented by a septum of radiating muscles with interposed 
sheets of connective tissue. The head of the notochord has a 
wide lumen and a strong cuticular membrane, into 
which these muscles are inserted. The neck and the 
corresponding portion of the skeleton as well asits 
diverging cruraareabsent. There are two proboscis 
pores, but they are very narrow,and do not appear to 
reach the cavity. A sense-organ in the shape of a pit in 
the mid-ventral line of the proboscis is present. 
