BY R. J. TILLYARD. 53 



these structures are undoubtedly artefacts, being nothing mure 

 nor less than a crystallisation of some ingredient used in the 

 preservation of the gills studied by Ris. 



Ris himself says of these structures: — I shall not disguise the 

 fact that my first impression on seeing them was that of an 

 artificial product, at first of crystals of a fatty acid, then of a 

 fungoid growth. Neither of these impressions stood the light 

 of clear observation, and I am convinced that we are dealing 

 with a certainly extremely curious structure peculiar to this 

 organ. 



The reasons for concluding that Ris' first impression was the 

 right one, i.e., thai, the fibril-tufts are artefacts, may be briefly 

 stated as follows : — 



1. They do not occur in any type of saccoid gill, so far studied, 

 which is known definitely to have been properly fixed, and to 

 have had the fixative completely extracted. 



2. They do not occur in the lateral abdominal gills of Pseiuio- 

 phcea, the internal structure of which, on Ris' own showing, is 

 a miniature reproduction of the saccoid gills as far as the alveoli 

 are concerned. 



3. They occur just in those parts of the organ (viz., the peri- 

 pheral alveoli) where they would be found if (a) the larva had 

 been partially fixed, and the fixative not washed out: or if ib) 

 the larva had been greatly over-fixed, so that only the more 

 open central portion, with the Vjlood-canals, had subsequently 

 been freed of fixative. 



1. The fact that fibril-tufts are found adhering to the wall of 

 the large blood-canal, and some even suspended in its lumen (see 

 the italicised portions of the quotation from Ris given above) 

 seems to me to be fatal to the argument that they are a special 

 or peculiar structure found only in this type of gill. Either they 

 are a formation produced within the alveoli as part of the living 

 cell (it is clear that Ris was inclined to regard each alveolus as 

 a single cell), or they are a foreign body. If the former, they 

 could not occur also on the wall of the blood-canal, or free within 

 its lumen; for there is no connection between the blood canal 

 and the alveoli. 



