MICROHYDRA DITRING I907. 97 



any given optical field with some rapidity, but by the use of 

 what machinery I have not determined. My earliest obser- 

 vations of twenty-four years ago, as recorded in a note book, 

 frequently refer to the crenulations of the ectodermal surface 

 near the extremities. One note in particular reads: " Num- 

 erous cell outlines appeared and disappeared, swelling and 

 contracting like the lobose pscudopodia of amceboids ; " 

 another entry runs, " there is a constant amoeboid change of 

 shape in the surface cells of the ectoderm, as well as a slight 

 writhing, expansion and contraction of the whole, so that 

 their change of position is quite obvious from one night to 

 the next." 



A FEW SIGHTS WORTH RECORDING. 



In describing a marine hydroid, also without tentacles, 

 found off the coast near Ostend by Dr. Greef, of Bonn, he 

 speaks of a "homogeneous cuticula " covering the basal 

 part of the ectoderm of Protohydra ; and something similar 

 may been seen in the drawings made by Messrs. Bourne, 

 Parsons and Fowler of the fresh water hydroids examined by 

 them in London, supposed to be the antecedent of the medusa, 

 Limiiocodiitm soiccrbii ; but I am far from convinced that 

 anything of the nature of a "tubular habitat" can be pro- 

 perly associated with the structure of Microhydra ryderi. In 

 common with snails and many worm-like forms there is a 

 mucous excretion from their bodies, and while fixed in posi- 

 tion upon stones, etc., this necessarily gathers about it par- 

 ticles of silt and other refuse matter that come in contact with 

 it. This, in time, may accumulate to some depth around Af. 

 ryderi, but it is altogether adventitious, and the tube or mass 

 is not utilized as a place of retraction, for protection, as is 

 the case with many worms, rotifers, etc. Here, on the glass, 

 the larvae and young hydroids are for a long time quite clear 

 and transparent ; but after feeding a while in the same place, 

 it would seem that the rejected matter thrown out through 

 the mouth is apt to find lodgment upon this excretion around 



