540 



G. 0. BOUENB. 



remnant of the primary body cavity existing between ectoderm 

 and endoderm. The muscles of the head of Lopadorhynchus 

 are derived from the body muscles, as is the case in Polygor- 

 dius and other Annelids. 



End. 



D.MJF! 



T.M.Fi 



Fig. 2. — Transverse section of an older larva, showing the further develop- 

 ment of the muscle-plates and the origin of the ventral ganglion chain, cirrhi, 

 and seta-sacs. B.C. Ectoderm. End. Endoderm. D.M.P. Dorsal muscle-plate. 

 V.M.P. Ventral muscle-plate. B.C. Dorsal cirrhus. V.C. Ventral cirrhus. 

 S.S. Seta-sac. V. gang. Ventral ganglion. V.S. Ventral shield. 



The seta-sacs are invaginations of the ectoderm, solid at 

 first, but subsequently developing a lumen, the cells lining 

 which secrete the setae. The sac of the aciculum is formed as 

 a special outgrowth of the seta-sac, which becomes partially 

 constricted off. 



The gonads of Lopadorhynchus are derived from the ecto- 

 derm. Earlier or later in development the ectoderm cells 

 lying behind and to the side of the parapodial ganglia prolife- 

 rate, and the mass of cells thus formed is invaginated in the 

 form of a pear-shaped body projecting inwards towards the 

 body cavity. In the female, cells are separated from the inner 

 ends of these ingrowths, which develop into ova, wander free 



