24 



Mr. W. Bateson. On the Development of [Dec. 18, 



ventral surface, immediately below the anus. The skin of this organ 

 is wrinkled by five to seven wrinkles, and is full of large glands, 

 probably secreting mucus. It serves as a strong sucker, by which 

 the animal can be kept in position. Its presence is possibly cor- 

 related to the fact that at this period of larval life the animals creep 

 up to the surface of the sand, protruding the conical proboscis at the 

 bottom of the shallow pools in which they live. As the heat of the 

 sun beating upon the sand-flats at low tide is very great, it is probably 

 important that the larvae should not be washed out of the pools by 

 the tide, and so dried up, which is prevented by the presence of this 

 sucker. The whole organ atrophies soon after seven gill-slits are 

 acquired, at which stage the body is long enough to be coiled round 

 foreign bodies. 



(3.) As the animal increases in size, the animal becomes more and 

 more flexed upon its ventral surface. This condition is never lost 

 throughout life, the body of the adult being twisted in a right- 

 handed corkscrew spiral, which cannot be completely straightened 

 without stretching the tissues. 



(4.) The constriction at the base of the proboscis increases until 

 the stalk in a specimen possessing four gill-slits is not more than 

 ■2-5-0 inch in thickness. 



(5.) In the third region of the body a constriction appears imme- 

 diately in front of the gill-slits. This constriction is especially 

 developed on the dorso-lateral aspects. It is posteriorly directed, 

 passing from above downwards. The skin of the region thus marked 

 out between the gill-slits and the original collar region becomes 

 thicker, and never acquires the transparency of the rest of the body. 

 At the period at which three gill-slits are present it grows backwards, 

 gradually forming an opercular fold over half the first gill-slit ; sub- 

 sequently in the adult it covers the three anterior pairs of gill-slits. 

 These gills thus open into a small atrial cavity. The skin of the 

 collar becomes filled with mucous glands, and acquires a bright 

 orange colour. The collar is thus made up of two portions of 

 different origin, and, as will be shown, its cavity is lined by meso- 

 blast derived from both the second and also from the third pair 

 of mesoblastic sacs. 



(6.) As a result of the deepening of the constriction between the 

 proboscis and collar, and by a forward growth of the lower limb of 

 the latter, the mouth is anteriorly directed when three pairs of gill- 

 slits are present. 



(7.) In larvse with 2 — 3 pairs of gill-slits, the first appearance of 

 the division of the alimentary canal into three regions may be observed. 

 This division is apparent as soon as the skin becomes transparent, and 

 is very conspicuous owing to the presence of a bright yellow-brown 

 secretion, probably of hepatic character, in the middle or stomach 



