[No. 7. OSLER — FRESH-WATER POLYZOA. 401 



in the method of growth, such variation from the ordinary 

 species described in our zoological text books. Happily 

 in the American Naturalist for that year we met witli Mr. 

 Alpheus Hyatt's papers on the Fresh-water Polyzoa, tlien in 

 course of publication, and obtained full information therefrom. 

 On examining the surface of a mass of Pectinatella the polyps 

 are seen to be arranged, as seen in the spirit preparation, in close 

 areola3, which, being crowded and compressed together, often 

 assume hexauoual outlines. From the periphery of these irregu- 

 lar areas the polyps project, the central part being as a rule un- 

 occupied. When in the water the protrusion of the innumerable 

 tentacles gives a fine velvety appearance to the surface, which 

 entirely disappears on touching the polyps or agitating the water. 

 This species is, however, much less timid tlian some others, and the 

 polyps over even a small mass do not all withdraw on a slight irri- 

 tation. The color of the polypidom is a light brown, or when the ten- 

 tentacles are extended, a faint rosy red hue, due to tiie color about 

 the throat, glimmering through them. Tow^ards the central part of 

 the areolae, white, brown and dark spots are seen, representing oval 

 at various stages of development. The coenoeeium composed of 

 the dense agoregation of polyps is closely united to the subjacent 

 gelatinous mass, which constitutes here theectocyst. On removal 

 of the polyps the surface of the jelly presents patterns correspond- 

 ins: to the arranoement of the animals, irregular areas grooved in 

 a radiate manner. The superficial portion of the ectocyst has 

 often a reddish tint, and the deeper parts slightly greenish from 

 the presence of a confervoid urowtli. Manv masses of lar!>e size 

 present a jelly perfectly colorless and pure throughout. Upon 

 the development of this jelly, wliieh is to be regarded as a definite 

 excretion or secretion from the animal, the size of the polyp masses 

 depends. When encrusting boards they are usually flat, larger 

 conoidal projections occurring at intervals. Around the stems of 

 rushes the most beautiful masses are found. Tiie small one be- 

 fore you gives a good idea of the graceful symmetry of the growth. 

 I have measurements ot such a symmetrical cluster about a reed 

 which was 14 inches in leni::th and 10 in circumference, the 

 weight 9 lbs. In some seasons the luxuriousne.ss of the irrowth 

 of these creatures is extraordinary. In the still quiet water in 

 the marsh on either side of Desjardin canal, just before it passes 

 through the Burlington heights, I have met with masses which 

 would not go into a pail. The largest I have ever seen lay at 

 ToL. X. y2 Ko. 7. 



