214 



INVERTEBRA TE MORPHOLOG Y. 



these cords at regular intervals, the lower ring being con- 

 nected with the cells which bear the protrochal cilia and 

 forming the prototroch nerve. 



At the apex of the lower cone and ventral to the intestine 

 lie two cells, or two masses of small cells, which constitute the 

 mesoblasts and give rise to two longitudinal mesoblast bands 

 (mb). A few scattered cells are also found between the ecto- 



m 



M 



mb 



FIG. 102. TKOCHOPHORE OF POLYGORDIUS (after HATSCHKK). 



A ~ anus. mb = mesoblast-band. 

 ap = apical plate. ne = uephridium. 



M == mouth. pro prseoral baud of cilia. 

 m, m', m" - muscles. po = postoral band of cilia. 



derm and the digestive tract, some of which elongate and be- 

 come muscle-fibres (m}, and which have been thrown off from 

 the mesoblast bands. In some forms a band of muscle-fibres 

 underlies the prototroch cells. In the neighborhood also of 

 the mesoblast bauds in the posterior cone there occurs on 

 either side of the digestive tract a small, sometimes branched, 

 tubular body, the head-kidney (ne). Each kidney consists of 

 a row of perforated cells, terminating in a funnel-shaped 

 structure closed at its mouth by a cell, the whole structure 

 thus agreeing closely with the uephridia of the Platyhelmiuths. 

 From such a larva the adult condition is derived by the 

 gradual elongation of the posterior part of the body an 

 elongation with which the mesoblast bands keep pace, the 

 mesoblasts retaining their position at the posterior extremity, 



