400 E. W. MACBRIDE. 
the entire absence of the lateral septum in the 
genital region,in the absence of genital wings, and 
especially in the structure of the central organs of 
the proboscis. ' 
The notochord is prolonged as a narrow solid cord 
of cells almost to the anterior end of the proboscis; to this 
corresponds a prolongation of the plate of dorso-ventral 
muscles. The pericardium and heart are each pro- 
longed into two great horns, which run alongside the 
notochord. It is from this circumstance that the name of 
the genus is derived. The mutual relation of these organs is 
shown by the series of sections represented in Pl. 29, figs. 
12 a—e. The pericardium has lateral as well as ventral 
muscles, and the circular muscles of the proboscis are very 
strong. 
The collar region has the skeletal rods reaching through 
two thirds of its length. ‘There is a large mass of chondroid 
tissue easily distinguishable from the primary skeleton, and for 
this reason the collar skeleton (Pl. 30, figs. 11 and 13) is of 
clumsy build. 
The central nervous system has no dorsal roots. 
The peripharyngeal cavities are confined to the 
lateral walls of the buccal cavity; hence their muscula- 
ture is only dorso-ventral instead of circular: it is completed 
above by transverse muscles belonging to the ventral wall of 
the perihemal cavities, and below by muscles derived from 
the collar celom; hence also there is noventral collar 
blood-plexus, since the peripharyngeal cavities do not meet 
ventrally. 
The inner gill-pores are so elongated as almost to meet in 
the mid-ventral line, leaving between them only (Pl. 29, 
fig. 14c) a strip of hypobranchial epithelium. Though there 
are no genital wings like those of Ptychodera, yet the body is 
much broadened in the branchio-genital region, and these 
lateral portions contain several rows of gonads. 
In the genital region there are a varying number of 
round pores opening from the gut to the exterior 
