780 CHARLES L. IIOULENGER. 



When we remember the similar origin of the two kinds of 

 cells from undifferentiated interstitial cells, we need not be 

 surprised that they both possess the same powers of active 

 movement. 



That the thread-cells are identical in origin with the sex- 

 cells is further emphasised by the fact that in exceptional 

 cases part of the testis of Moerisia can give i-ise to a nemato- 

 cyst battery instead of producing sperm-cells, as shown in 

 PI. 43, fig. 12. 



Both kinds of cells are lirst to be recognised in the endo- 

 derm of the medusa-bud ; this does not necessarily imply 

 that they originate in that layer; in my account of the early 

 development of the gonophore, I showed that intei'stitial 

 cells of the ectoderm occasionally migrate through the 

 structureless lamella of the hydranth and become incorpor- 

 ated among the proliferating cells of the endoderm. It is 

 probable that these cells or their derivatives give rise to the 

 sex-cells and nematoblasts. 



In my description of the anatomy of Moerisia lyonsi (5), 

 I mentioned that exactly the same types of nematocysts 

 were to be found in the medusa as in the hydioid ; in this 

 paper I hope to have proved that they are not only identical 

 in structure, but actually originate from the same cells. 

 This fact is one which might be of use in systematic work on 

 the HydromedusEe, where the assignment of Medusae to 

 hydroids is often only a matter of inference ; a careful com- 

 parison of the nematocysts of the two stages should be of 

 great value in this connection. 



5. General Conclusions. 



(1) The stinging-cells of the medusa of Moerisia lyonsi 

 are not developed "in situ " on the principal batteries, but 

 migrate to their final positions on the oral lip, or on the 

 tentacles. 



(2) The stiuging-cells of the oral battery are formed in the 

 endoderm of the manubrium, just below the stomach ; those 



