No. 463] WORTH AMERICAN INVERTEBRATES. 427 
with the blood vessels in the esophageal region. One or more 
efferent ducts lead from each main canal to the exterior of the 
body or, in rare instances, to the esophagus. 
The muscular system consists of either two or three strong 
layers of fibers, the arrangement of which forms a convenient 
diagnostic character and is described in the key to the orders. 
In some forms there are additional secondary layers. 
The relative position of the principal nerve cords with respect 
to the muscular layers is likewise of primal importance in clas- 
sification. The central nervous system consists of a four-lobed 
brain, the two lobes of each side being closely united with each 
other and joined to those of the other side by a commissure 
above and one below the rhynchodzeum, and a pair of large lat- 
eral nerves, accompanied by ganglion cells, extending from the 
ventral pair of brain lobes to the posterior end of the body. In 
addition, a dorso-median nerve is commonly present, and some- 
times a ventro-median one ; most forms have a pair of well devel- 
oped esophageal nerves, and nearly all have special proboscis 
nerves, together with peripheral nerves to the integument, 
ocelli and other sense organs. In most forms a pair of highly 
specialized cerebral sense organs lie in close proximity to the 
dorsal brain lobes, with which they are completely fused in the 
Heteronemertea. A pair of lateral sense organs occur on the 
lateral margins of the body in the nephridial region in many 
Paleonemertea ; frontal sensory pits are found on the tip of the 
snout in many forms, while Carinoma has a number of sensory 
pits on the dorsal aspect of the head. Ocelli are often present 
in considerable numbers, but are wanting in the Paleonemertea 
and in many species of the Heteronemertea. 
The sexes are separate in most species, although a few are 
hermaphroditic. The sexual products develop in simple sacs 
in the body parenchyma and are discharged directly upon the 
surface of the body. Where lateral intestinal lobes are devel- 
oped the gonads often alternate with them with much regu- 
larity. 
In the larval development, some forms (Lineidz) pass through 
a complicated metamorphosis, with the formation of a free-swim- 
ming larva of highly specialized form (pilidium, or Desor's larva), 
