NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 23 



usually bears a tuft of cilia, and with which there may be associated a 

 pair of small larval tentacles, and sometimes a pair of "eye spots." 

 From tne apical ganglion, nerves radiate posteriorly (LNv) on the 

 inner surface of the epidermis, and these longitudinal radial nerves 

 are connected by bands of circular fibers (CNv), chief of which is 

 the nerve ring of the prototroch (Prtr). The nerve tracts, both radial 

 and circular, closely follow the peripheral muscle bands of the larva 

 (fig. 8), thus attesting that the nervous and contractile elements arose 

 from common ectodermal neuromuscular rudiments. The nerve tissue 

 is situated between the muscle fibers and the epidermis, the nerve cells 

 being scattered individually, or condensed in small ganglionic groups. 



The nervous system of the polychaete trochophore is best known 

 from the elaborate studies of Kleinenberg (1886) and of E. Meyer 

 (1901) on the larval development of Lopadorhynchus, a small errant 

 polychaete of the family Phyllodocidae (fig. 13 D) having two pairs 

 of prostomial tentacles but no palpi. The larva of Lopadorhynchus 

 is a typical trochophore (fig. 8) with an equatorial band of cilia, the 

 prototroch, just above the mouth. The apical ciliary organ, however, 

 does not have the usual form and position ; it is transposed to the 

 anterior ventral surface, and is divided into a well-developed organ 

 on the left (B, So), and a rudimentary organ on the right (so). The 

 episphere contains seven pairs of longitudinal nerves (w^-n^), and is 

 encircled by three nerve rings {rno^-rno^) above that of the prototroch 

 (Rn). In the hyposphere there is but a single nerve ring (A, rnu). 

 The largest of the longitudinal nerves are two thick lateroventral 

 nerve tracts (B, «-), which anteriorly (apically) are continuous with 

 each other in a wide transverse commissural arch within the episphere, 

 and posteriorly are extended into the hyposphere as a pair of large 

 lateral trunks {Vdn) that break up into smaller branching nerves. 



The neural cells of the Lopadorhynchus larva are described in great 

 detail by Meyer. In general they lie along the fiber tracts (fig, 8 B, 

 NCls), where many of them are aggregated into small ganglionic 

 clumps, particularly in the episphere. In the early stages of develop- 

 ment, according to both Meyer and Kleinenberg, the neurocytes are 

 generated from the ectoderm in association with muscle cells, and the 

 principal neuromuscular rudiments of the episphere represent larval 

 sensory organs (fig. 16 A), of which the nerve cells {n) form small 

 ganglionic centers. The scattered neurocytes are probably the gener- 

 ative cells of the fibers in the nerve tracts. The ganglionic centers of 

 the larva pertain to the apical ciliary organs, a pair of transient larval 

 antennae, the two pairs of persistent tentacles, which are dorsal and 

 ventral in the adult (fig. 13 D), and the nuchal organs, but include 



