210 ARTHUR DENDY. 



and all together form a single-layered, continuous epithelium 

 over the outside of the Ascon-tube. As a rule, in ordinary 

 preparations, although the nuclei and granules of the ectoderm- 

 cells may be clearly enough visible, it is very difficult to dis- 

 tinguish the outlines." 



In writing the above I regret to say that I overlooked 

 Metschnikoff^s very precise and clear account (28) of the 

 ectoderm of Leucosolenia (Ascetta), published long before, 

 in which he describes and figures an ectodermal epithelium 

 precisely similar to that previously described by Schulze in 

 Sycon raphanus, and subsequently by myself in Grantia 

 labyrinthica and Leucosolenia Wilsoni. In the same 

 paper (28) MetschnikoflP also states that the ectoderm of 

 Leucandra aspera has the same structure. 



Metschnikoff's latest views on the subject of the ectoderm in 

 sponges in general are contained in the following very interest- 

 ing passage from his recent work on ''Inflammation " (29). 

 After pointing out that the body of the sponge is composed of 

 three characteristic layers, he observes : " La couche super- 

 ficielle, ou Fectoderme, revet le corps entier de cellules 

 epitheliales plates, limitees entre elles par des contours qui 

 deviennent tres nets apres Fapplication d'une solution de 

 nitrate d'argent. Les cellules memes sont visiblement con- 

 tractiles^ ce qui s'observe surtout aux bords libres des jeunes 

 individuSj ou on apergoit des prolongements amiboides ap- 

 partenant aux elements ectodermiques. La contractilite de 

 ces cellules joue certainement un role dans le phemomene 

 remarquable de I'ouverture des pores nombreux, eparpilles sur 

 la surface de I'eponge entiere, et apparaissant entre deux ou 

 plusieurs cellules plates.'^ 



Minchin also (25, 26) concludes that the normal condition 

 of the ectoderm in Leucosolenia clathrus, the form 

 specially studied by him, is flattened and plate-like^ and he 

 also ventures upon the generalisation that " the contractile 

 elements in all cases are the flattened ectodermal epithelium." 

 The contractile nature of the ectoderm- cells I do not doubt; 

 I believe it to be an important character, and I shall presently 



