CEPHALOCHOKDA 151 



ventral surface. During the growth of the larva the dis- 

 tortion increases and the gill is carried to the right side. The 

 first gill is followed by the appearance of a succession of gills, 

 and the fifteen gills thus formed all belong to the left side, but 

 are actually on the right side of the body. Later, but during 

 the formation of the series of the gills of the left side, the gills 

 of the right side begin to appear. They are formed high up 

 on the right side above the left gills, and the primitive median 

 line lies between the two sets of gills. As the right gills grow 

 they press the left series of gills around the ventral aspect of 

 the body to the left side. Only eight gills are developed on 

 the right side, and the first of the left series, together with the 

 last six, are suppressed, so that eight pairs occur. These 

 primary gills are widened and divided by tongue bars into 

 two each. 



Two further organs are developed in association with the 

 anterior endoderm. One of these is the club-shaped gland 

 which arises as an outgrowth of the endoderm on the right 

 side, opposite the dorsal margin of the mouth. It forms a 

 tube which extends downwards and forwards to an opening 

 which it makes in the ectoderm on the left side in front of the 

 mouth. The tube is therefore open at both ends. It has 

 only a temporary existence, and disappears with the first gill 

 of the left side. The morphology is not at all clear. The 

 second organ originates as a V-shaped thickening of the front 

 endoderm, the angle of the V being directed to the actual 

 median line between the two sets of gills. It is from the 

 beginning glandular and ciliated, and the thickening is 

 extended backwards as the endostyle. From the position of 

 the primitive endostylar modification of the endoderm the 

 actual mid-ventral line is seen to be high up on the right side, 

 and it is possible that the club-shaped gland represents in a 

 fleeting manner the left collar cavity, or hydrocoel, of the 

 Echinoderms. It tends to encircle the mouth as does the 

 hydrocoel of the Echinoderm, and it establishes a communi- 

 cation with the exterior, and the mouth as in the Echinoderms 

 is on the original left side. 



After about three months of larval life the asymmetry is 

 gradually corrected. The gills occupy normal positions on 



