462 E. KAY LANKESTER AND ARTHUR WILLEY. 



pharynx. It must be pointed out that the sections are com- 

 plicated and rendered a little difficult of interpretation at first, 

 by the fact that the margins of the gill-slits are irregularly 

 curved and folded, so that they cross the plane of section, and 

 (as in fig. 7, PI. XXX) the slit itself becomes divided in the 

 section by a part of the projecting margin. A further modi- 

 fication in appearances is due to the greater or less opening of 

 the gill-slits, which can be varied by muscular action during 

 the life of the animal. The atrial tube or cavity is also found 

 to vary in size and dimensions as soon as it is formed, owing 

 to the varying extension or contraction of its muscular floor 

 formed by the union of the sub-atrial ridges (compare fig. 12, 

 PI. XXX, and figs. 18 and 20, PI. XXXII). As was pointed 

 out by one of us in the case of adult Araphioxus distended 

 with genital products (see Lankester, ' Quart. Journ. Micr. 

 Sci.,' vol. xxix, PI. XXXV, fig. 4), so here in the larva the 

 atrium can be distended to such an extent as to practically 

 Y obliterate the metapleural ridges and their lymphatic canals, 

 which reappear when the distension ceases. 



The description of the individual figures seriatim will be 

 found, it is hoped, sufficiently explanatory of points which 

 have not been specially mentioned in the general body of the 

 memoir. 



