BY WILLIAM J. BYRAM. 23 



filaments, instead of radiating from the main axis, grow down- 

 wards upon it and form an envelope, closely investing it. Here 

 -we have a step towards differentiation, or division of labour, for 

 these investing cells are no longer independent, but constitute a 

 membrane foreshadowing the cuticle or cortex of the higher 

 plants. Another illustration is afforded by the higher forms of 

 sea weeds, where we meet with a faint hint of the distinction 

 between leaf stem and root. The photomicrograph shows the 

 cells of the frond of the beautiful Polysiphonia. These are 

 vegetable types, but differentiation in the animal follows a similar 

 course. From single cells like the sun-animalcule we pass to 

 groups like the vorticella, and thence to colonies of animals. But 

 still each cell lives for itself alone ; there is no division of labour. 

 Go a step forward, however. There is a little creature known as 

 the hydra, often found in ponds amongst duckweed and 

 utricularia, which shows in a most decided manner the early 

 advance in differentiation. I have often met with it in the ponds 

 in Bowen Park. It consists of a cylindrical body, ending in a 

 small orifice, and crowned with from six to eight tentacles, with 

 which it captures minute creatures for its food. As we watch the 

 hydra we observe that it assumes so many different shapes that 

 if you did not see it passing from one to the other you would not 

 connect them with the same animal. Sometimes it is an almost- 

 spherical mass, and the tentacles are reduced to small rounded 

 excrescences ; sometimes it is fully expanded and the tentacles 

 are thin, delicate processes. Between these extremes every 

 gradation occurs. The photomicrograph on the screen shows it 

 about half expanded. The diagram presents it in its fully 

 expanded condition. In structure the body of the hydra consists 

 of but two layers of cells, an outer and an inner, but the inner 

 cells have taken upon themselves the function of nutrition, and 

 the outer cells are both irritable and contractile, forming a kind 

 of rudimentary nervous and muscular system. In certain of the 

 outer cells there is a strange and deadly weapon. If we tear a 

 hydra to pieces with very fine needles and examine the pieces 

 carefully with sufficient magnification, we see that certain of the 

 outer cells possess peculiarities. They exhibit a clear elliptical 

 cavity. Coiled up within this cavity, like a spring, is a delicate 

 thread, furnished at the basal end with three projecting barbs. 

 The cavity is filled with a poisonous fluid, though what its 

 chemical nature may be I have never been able to determine. 



