MIOROHYDRA RYDERI. 627 



showing manner of eversion of hypostomal organ, in longi- 

 tudinal section.^' — J. A. R., X 365. 



Fig. 20. — " Oblique section of pedal disc of adult, showing 

 cuticular sheath." — J. A. R., X 365. 



Fig. 21. — Original diagram by the writer to show the 

 locality of the larval bud (Z.&.), its gradual growth, and the 

 manner of sepai*ation of the fully formed larva. A course of 

 24 hours or less. 



Fig. 22. — Highly magnified section (X 244), showing bud- 

 ding larva {l.h.). 



Fig. 28. — Another bicapitate hydroid, showing larval bud 

 in situ, lasso threads highly stimulated. X 67. 



Fig. 24. — Young larva forming capitulum ; thread-cells 

 accumulating at distal end. X 112. 



Fig. 25. — Single hydroid adherent to glass of jar; base 

 and adjacent parts showing adherent threads of Nostoc and 

 adventitious particles. X 30. 



Fig. 26. — Drawing by J. P. Moore from ci'oss section of 

 mature hydroid by Harold Wingate, showing structure, en- 

 teric folds, etc. Magnified 480 diameters. 



In the latter part of January, 1885, while examining under 

 the microscope the surface of some small fragments of gneiss 

 and mica-schist, chipped from larger rocks in the bed of 

 Tacony Creek, a small mill-stream near Philadelphia, Penn., an 

 affluent of the Delaware River, I first observed the primitive 

 freshwater hydroid, since classified as M. ryderi. The 

 stones had been brought into my house during the previous 

 summer to enable me to study the winter condition of a 

 newly discovered bryozoan, Paludicella erect a, mihi 

 (since named Pottsiella e recta by Prof. Kraepelin), that 

 grew abundantly upon them. 



The polyp, as seen, was about one half a millimetre in 

 length by one tenth of a millimetre in thickness, nearly 

 cylindrical, sometimes simple, in other instances divided near 

 the base of support into two nearly equal, divergent branches. 



