xvi ILLUSTRATIONS. 



diatoms are Coscinodiscus radiatus. The first ring 

 cluster is wheel plates found in the skin of the worm 

 Chirodota. The second and fourth circles are the 

 diatoms Actinocyclus Ralfsii. The third circle is the 

 diatom Arachnoidiscus ETirenbergii. The outside 

 cluster is composed of the anchors and perforated anchor 

 plates found in the sldn of the worm Synapta, as also 

 of the long diatoms Pinnularia virides. The nega- 

 tives for this and the following prints of micro-photo- 

 graphs were taken for me by W. H. Walmsley, of 1016 

 Chestnut Street, Philadelphia. Pages 164 and 125 



PLATE VIII. OPPOSITE PAGE 128. 

 BIRDS HEAD POLYZO A AVICULARIA. 



(MAGNIFIED ABOUT THIRTY DIAMETERS.) 



One of these illustrations is directly from nature, and the 

 other is from a drawing of a stem of the zoophyte, 

 showing the animals with tentacles extended, and the 

 muscles which actuate the birds head attachments. 

 These miniature heads, of no use or purpose in the 

 colonial economy, so far as known, keep up a continual 

 biting and snapping as long as there is life in the animal 

 which they seem to guard. Page 126 



PLATE IX. OPPOSITE PAGE 136. 

 SPIRAL TISSUE IN LEAF STEM OF CASTOR OIL PLANT. 



1st. Transverse section, magnified 12 diameters, showing the 

 ends of the spirals in what are called the fibro-vascular 

 bundles. 



2nd. Longitudinal section, magnified 70 diameters, showing 

 the spiral vessels as they lie in one of those bundles. 



These little tubes, made, in the case of this plant, of closely 

 coiled fibers, are the passage-ways through which the 

 sap circulates to the extremity of the leaves and back 

 again. In the leaf -stem they are laid away in small 

 clusters between the wood cells and the pith cells. 

 There are over 400 of these spiral vessels in the stem 

 from which this specimen is taken. They are not larger 

 than human hairs, and yet they are wound with a thread 

 inconceivably smaller, and are as perfect as coils of wire 

 around a form. Pages 135 and 145 



