KI.E1NENBERG ON DEVELOPMENT OF LOPADORHYNCHUS. 535 



successive differentiation of derivatives of the two primary 

 layers, ectoderm and eudoderm. 



In the first place it may be stated that the adult Lopador- 

 hynchus has no blood-vascular system, and Kleinenberg was 

 unable to find its nephridia. The youngest larva observed 

 had a well-developed provisional nervous system, consist- 

 ing of a ring of nerve-fibres lying beneath the prototroch, 

 and a series of ganglion-cells, connected by fine processes with 

 the sub-prototrochal nerve-ring. It is unnecessary to enter 

 into the exact arrangement of these ganglion-cells : they lie in 

 the ectoderm of the preoral lobe of which they are differentia- 

 tions, and although the cephalic ganglion of the adult animal 

 and the sense-organs in connection with them are developed 

 from the ectoderm of the preoral lobe, the central nervous 

 system of the trochosphere has but very little share in their 

 establishment. The whole of the nervous structures of the 

 head of the adult are formed from three centres : firstly, from 

 nerve ganglion-cells, which make their appearance at an early 

 stage in the ectoderm of the preoral lobe ; secondly, from parts 

 of the sense-plates already mentioned ; thirdly, from indif- 

 ferent ectoderm cells. The more anterior of the two pairs 

 of prominences, which have been described as appearing on 

 the ventral surface of the preoral lobe, and are called by Klein- 

 enberg the pole antennas (Scheitelantennen), prove to be 

 nothing more than cell-proliferations, and eventually form the 

 anterior lobes of the cephalic ganglion of the adult. The 

 remainder of the ganglion is formed from ganglion-cells and 

 nerve-fibres which arise in connection with the sensory organs 

 of the head (the two pairs of antennae, the olfactory pits, &c), 

 to which are added cells derived independently from the ecto- 

 derm. The newly-formed cephalic ganglion is first placed in 

 connection with the sub-prototrochal nerve-ring by means of 

 commissural fibres ; the angles of these commissures are after- 

 wards produced posteriorly into the oral lobe, and, becoming 

 subsequently connected with the anterior ganglion-pair of the 

 ventral chain, form the pericesophageal connectives. In a 

 later stage of growth the cephalic ganglion retires from the 



