THE STARFISH SOLASTER ENDECA. 23 



These points hardly require further description. Emphasis may, however, again be 

 laid on the fact that while the larval sucker and arms are actively absorbed, much of 

 the preoral lobe epiderm is not destroyed, but is incorporated with the oral surface 

 of the Starfish. 



The most important new structures that appear externally during this period are the 

 eye-spots, the mouth, and the anus. 



The Eyes. — In the first seven rays the eyes appear as bright reddish spots at the 

 base of the terminal tentacles several days before the formation of the mouth. In 

 rays VIII and IX they appear respectively along with and a little later than this 

 opening. After death their pigment is extremely soluble and readily washes out, so 

 that they may easily be overlooked in their earliest stages unless they have been 

 observed in the living condition. 



Formation of the Mouth. — The ring-canal of the hydrocoele now encloses a relatively 

 large central area on the oral surface. The mouth appears in the middle of this area, 

 usually in the form of a triradiate fissure (PI. I. fig. 13), the angles of which point 

 approximately in the direction of interradii II/III, V/VI, VIII/IX. Later, the opening 

 becomes circular with slightly crenated edges. That part of the body-wall which lies 

 between it and the ring-canal forms the soft membrane of the peristome. Still later, 

 the first pair of adambulacral ossicles in each interradius comes to project inwards 

 towards the centre of the mouth, carrying spines of larger than average size. The 

 mouth-armature is accordingly of the adambulacral type, although the first pairs of 

 ambulacrals do not recede so markedly as in the better-marked examples of this type. 



Formation of the Anus. — The formation of the anus takes place several days after 

 that of the mouth. The opening is small, and is to be found near the centre of 

 the aboral surface, in the interradius which is designated V/VI according to our 

 system of numbering. It will be remembered that in the ordinary five-rayed Starfish 

 the anus, when present, falls within interradius V/I according to the same system. 

 We can say, then, that the anal interradius of Solaster corresponds with that of an 

 ordinary Starfish, on the assumption, which we have already seen to be well grounded, 

 that rays VI to IX are later additions or interpolations. 



In this connection it is worthy of note that in Solaster jpapposa, which has a greater 

 number of rays than our Solaster^ the anus is still to be found in the same interradius, 

 namely V/VI. Equally striking is the fact, which I have been able to establish for both 

 species, that in any individual, no matter whether the number of rays is greater or less 

 than the normal, the anus still occurs in that interradius, which, according to our 

 method of numbering, is the normal one (8 a). The principle that parts forming a 

 serial succession are specially liable to variation in number may perhaps serve to throw 

 light on the very great variability of both species, and particularly of S. papposa (17), 

 in the number of their arms. 



For mesenteric relations of rectum and anus, see p. 39. 



In %he weeks that succeed the formation of the mouth and anus, the most marked 



