CILIARY MECHANISMS ON THE GILL IN AMPHIOXUS. 43 



into a small compass so as to leave the inhalent and exhalent chambers 

 continuous. 



The characters of a transverse section of a gill-leaflet of Solenomya 

 •can now be recognized. Eidewood (p. 193) in describing the ciliation 

 of the gill remarks, " Cilia are confined to the thickened edge, and appear 

 to form a continuous investment of it, not differentiated into frontal 

 and lateral tracts. The material available is not sufficiently well pre- 

 served to allow of a definite statement upon this point, but it is worthy of 

 remark that no gap between the frontal and lateral cilia is shown in the 

 figures (of the transverse sections) given by Pelseneer, Kellogg, and 

 Stempell. The cilia extend as far as the level of the outer edge of the 

 thickened chitin bands, and the shortest are those on the frontal surface. 

 Pelseneer shows large latero-frontal cilia, but this is possibly an error." 



The foregoing examination of the living leaflets shows that Pelseneer's 

 depiction of large lateral cilia (the latero-frontal cilia) is correct, and that 

 all figures err in depicting a continuous covering of cilia at the tip of the 

 gill (see Fig. 10), thus justifying Ridewood's cautious remarks in this 

 respect. A transverse section may or may not show cilia on the ab-frontal 

 face according as the section passed through a ciliated knob or not. 

 An attempt will be made to make preparations from well-preserved gills 

 and to figure a transverse section. 



PtESP:MBLANCE OF THE CILIATION OF THE GILL 

 IN SOLENOMYA AND NUCULA. 



From the above description of the ciliation of the gill of Solenomya 

 it will be seen that there is a close resemblance to that obtaining in 

 Nucula (see Fig. 11). If the frontal surfaces of these gills be compared a 

 complete resemblance in this part is seen; frontal, latero-frontal, and 

 lateral cilia occurring in both and having the same function in both. The 

 ab-frontal cilia of the gill of Nucula, however, are absent from that of 

 Solenomya, and are replaced by ciliated knobs. The small patch of inter- 

 locking cilia on the lateral faces of the leaflets of Nucula have not been 

 seen in Solenomya, but a patch of interlocking cilia is present on the 

 ventro-lateral portion of the inner leaflets of both gills (see Figs. 11, c.d., 

 and 10, C.V.). Cilia interlocking with the mantle are present in both forms 

 at the tips of both leaflets ; those in Solenomya are, however, nearer to 

 the frontal surface than in Nucula in correlation with the difference in 

 position of the gill in the former. On the whole, therefore, the ciliation 

 •of the gill of Solenomya bears a very close resemblance to that of Nucula. 



