52 W. A. HERDMAN. 



which open at the upper end to the external world, and at the 

 lower into the extremity of the hepatic diverticulum. 



This state of affairs was long ago pointed out by Alder and 

 Hancock^ as seen in transparent specimens, and it has more 

 recently been demonstrated by Bergh in Phidiana Selencte, 

 Facelina Janii, Chlamylla borealis and Gonieolis 

 typica; but the communication has often been denied or 

 doubted, and Ray Lankester probably expressed the mental 

 attitude of most zoologists towards the matter when he wrote 

 in 1883 that the supposed communication of the hepatic caeca 

 in the dorsal papillse or cerata of some of the Ceratonota with 

 the exterior by means of apertures in the apices of the papillae 

 ''requires confirmation.^^ ^ Last year Mr. Clubb and I de- 

 scribed and figured^ sections showing the exact manner in 

 which the communication takes place in specimens of an Eolis 

 from the Puffin Island Biological Station, and 1 now give 

 some more detailed figures here (PI. X, figs. 32, 36, and 37). 



The upper end of each of the hepato-cerata is occupied by a 

 sac containing a number of large cells (the cnidocysts) filled 

 r^ with cnida or thread-cells. This cnidophorous sac is evidently 

 I ; an invagination of the ectoderm, the cnidocysts being modified 

 ectoderm cells (figs. 32, 36), and it communicates with the 

 exterior by a small but perfectly distinct and clearly-defined 

 aperture at its apex, through which the thread-cells are some- 

 times found protruding (fig. 36). 



The size and shape of the cnidophorous sac varies in different 

 species. Figs. 28a to 28c represent the upper ends of hepato- 

 cerata from Facelina drummondi where the sac is greatly 

 elongated, may become irregularly shaped, and overlap the 

 upper end of the hepatic caecum (fig. 80). The cnida are 

 ovate or nearly spherical in shape (figs. 30, 38, and 39), with a 

 small terminal projection, and the everted threads bear some 

 large spines arranged in a spiral round the base (fig. 39)^ and 

 smaller ones projecting alternately from opposite sides all the 



^ 'Eay Soc. Monograph,' part iii. 



" Article " Mollusca," 'Eucy. Brit./ ninth edition, vol. xvi, p. 059. 



'<* ' Proc. Biol. Soc. Liverpool,' vol. iii, p. 233, and pi. xii. 



