188 INVERTEBRATE MORPHOLOGY. 



front of the middle of the trunk region of the body. This 

 ganglion gives off numerous nerves, among which are two 

 principal nerve-cords passing backwards and giving off along 

 their entire length finer nerves which branch and finally lose 

 themselves in a fine ectodernial nerve-plexus throughout 

 which ganglion-cells are scattered. In addition to these 

 ectodernial portions three pairs of ganglia are found in the 

 head region at the sides of the oesophagus, the largest gan- 

 glion on either side being united with the supracesophageal 

 ganglion by a commissure. From the supracesophageal 

 ganglia, behind the commissures to the ventral ganglion, a pair 

 of nerves pass backwards to the two eyes (oc), which lie com- 

 pletely imbedded in the ectoderm of the dorsal surface of the 

 head, each consisting of three biconvex lenses imbedded in a 

 central pigment mass and surrounded on their outer surfaces 

 by a retina composed of an outer layer of cubical cells, a 

 middle layer of cylindrical cells with large nuclei, and an 

 inner layer of rod-like structures arranged perpendicularly to 

 the surface of the lenses. Behind the eyes lies a circular band 

 of fine columnar ciliated cells (o), which is supplied by a pair 

 of nerves arising from the supracesophageal ganglion be- 

 tween the optic nerves. The function of this organ is doubt- 

 ful, though it has been considered olfactory. Scattered some- 

 what regularly over the body are numerous round or oval 

 eminences consisting of a number of central spherical cells 

 arranged in two rows and bearing rod-like bristles. These 

 are enclosed in a sheath of cylindrical cells and below come 

 into contact or are continuous with terminal nerve-branches. 

 These sensory hillocks are supposed to be tactile in function 

 and resemble not a little the lateral sense-organs of certain 

 Annelids (see p. 210). 



The Chsetagnatha are without exception hermaphrodite. 

 The ovaries (ov) are cylindrical bodies lying in the trunk re- 

 gion of the body, one on each side of the digestive tract, and 

 upon the outer side of each is a tubular oviduct which ends 

 blindly anteriorly and opens posteriorly at the sides of the 

 body near the dissepiment between the trunk and tail regions 

 of the body. There is no communication apparently between 

 the cavity of the oviduct and the ovary or crelom, and the 



