254 



Prof. P. M. Duncan on Thallophytes 



* Physiologia Generalis ' o£ Saprolegnia ferax is very close (Plate 7. 

 figs." 36, 37, 38, 40, 41). 



There is an interesting point about this Achlyan from the English 

 littoral zone, and that is its resemblance in tint to those of the deeper 

 sea. The delicate sap-green of the homogeneous viscid granuleless 

 refractive cytioplasm is evident enough. Many filaments, however, are 

 colourless. 



X. Summary. 



Quekett, Eose, "Wedl, Kolliker, and Moseley have noticed and 

 described the borings of vegetable parasites in moUuscan shells, fish- 

 scales, and corals ; but no special attention has been paid to the filaments 

 penetrating the last-mentioned organisms. 



Corals from the littoral zone dowTi to 1095 fathoms are frequently the 

 seat of the parasitic growth of two kinds of Achlyce, whose horizontal 

 range is from Davis Straits to the tropics and 15° S. lat. 



Possil corals of Silurian age were also affected by closely allied, if not 

 specifically identical, growths. 



The method of investigation is by making thin sections of the scleren- 

 chyma, and also by dissolving out the carbonate of lime. 



The parasites are filamentous, and fill up the canals which they form ; 

 they resemble a mycelium, and penetrate the coral, living upon the 

 organic basis, and having their length, breadth, and straightness, or 

 branchings, dependent on the peculiar nature of the arrangement of the 

 spicula in the different species of the Madreporaria. The entry is made 

 from oospores, zoospores, and by the accidental contact of the parasites 

 whilst perforating algae situated on the wall of the coral ; and the pene- 

 tration and growth appear to be the combined results of the formation of 

 a soluble bicarbonate of lime by the action of carbonic-acid gas evolved 

 from the growing end of the tubular filament, of the pressure incident to 

 growth, and of the movements of the cytioplasm and the ceU-waU. 



The vegetative life of the parasites is accompanied by reproductive 

 efforts within the corallite ; for the aggregation of granules within the 

 viscid transparent cytioplasm can be detected, and their formation into 

 large conidia and into small unciliated zoospores also. 



Pollo\\dng the peculiar physiological habit of the Saprolegnian group of 

 Achlyce, the reproductive elements germinate and produce either large or 

 very small tubes which, after penetrating the parent ceU-wall, get through 

 the solid investment, and become indistinguishable from the filaments 

 derived from spores attached to the outside. 



The diameter of the largest canals containing filaments in which there 

 is occasionally a doubtful dissepiment, and which flourish in the organic 

 matter between the laminss of a septum, is inch ; that of the typical 

 and ordinary tubes is from j-^^ to inch ; and the finest tubes are 

 as small as 00,000 i^^^ ^'^ diameter. 



