24 DE. J. F. GEMMILL ON THE DEVELOPMENT OE 



new feature in the external appearance of the young Starfish is that the spines become 

 relatively very numerous and very prominent. The length of a single spine may 

 amount to about one-eighth of the diameter of the whole Starfish including the arms. 

 The spines now tend to become grouped in twos or threes, the members of each group 

 being united by an arching fold of integument which reaches about halfway up their 

 sides. It should also be noted that meantime a distinct notch has made its appearance 

 at the tip of each arm outgrowth, and that in these regions the spines become disposed 

 in two sets. It will be seen afterwards (p. 42) that in Solaster the skeletal plates 

 occupying the position of terminals are double from the first, the members of each 

 pair being an appreciable distance apart (PL I. fig. 8). 



Movements. — "J'he young Starfish may begin to move about with the help of its first- 

 formed sucker-feet during the sixth week, and therefore prior to the formation of the 

 mouth. In one of the small aquaria I had with me in Glasgow I marked the position 

 of eight of them at this period by pencilling circles on the outside of the glass. Two 

 days aiterwards all but three had shifted their position, some of them by as much 

 as two and a half inches. The young Starfish are now able to right themselves 

 after being turned upside down exactly in the manner of the adult. They cling 

 tightly to suitable surfaces, and if sucked up into a pipette or tube must be released 

 at once, as it will be difficult to get them out alive and uninjured should they succeed 

 in taking hold inside. 



After the mouth is formed the young Starfish wander about freely over the sides 

 and bottom of the tanks ; but perhaps the most interesting point to be noticed in their 

 habits is the facility with which they can " swim " in any direction just under the 

 surface of the water. Movement is fairly rapid, especially where there happens to be a 

 fine powdering of dust on the surface by which the surface-tension is increased. The 

 rate, as measured in half a dozen specimens of the age of six months and 2-76 mm. in 

 diameter, varied from 2b mm. in ten minutes downwards. The young Starfish floats by 

 keeping the tips of a number of its tube-feet protruded slightly above the surface. 

 Some of these are continually being withdrawn, advanced, and again protruded. With- 

 drawal is effected by means of a sharp muscular twitch, which pulls the tip at once 

 below the surface-level. On the other hand, in protrusion, the surface-film seemed to 

 shed off naturally from the tips of the gently extended feet. 



The tube-feet cause progression not by taking a sucker-like hold of the surface-film 

 as in ordinary movement over solid surfaces, but rather by elbowing the body along, 

 advantage being taken of the resistance due to surface-tension. At any given time 

 the tips of a considerable number of feet are protruded, and these have no difficulty 

 in keeping the Starfish afioat ; but as its specific gravity is now much greater than that 

 of water, owing to the formation of the skeletal plates, the animal will sink very 

 rapidly to the bottom if once it be depressed ever so little below the surface. To 

 begin with, it must have launched itself by crawling up the side of the tank, then 



