THE STAEFISH S0LA8TEE ENDECA. 39 



what will afterwards be recognisable as the stomach of the Starfish, and must not be 

 confused with the paired radial caeca, which are of still later development (see below). 



The Mouth. — The formation of the mouth has already been referred to (p. 23) as 

 taking place in the centre of the oral surface shortly after the sixth week of develop- 

 ment. In this region the wall of the enteron fuses with the body-wall and both 

 become thinner. The mouth-opening then breaks through, and at first the pharynx 

 is triradiate in cross-section. The angles point approximately to interradii II/ITI, 

 V/VI, and VIII/IX. There is no stomodseum. 



The Anus. — The anus is formed during the seventh or eighth week, in interradius 

 V/VI. An outgrowth from the enteron fuses with the body-wall, the actual opening 

 arising afterwards by rupture as in the case of the mouth. The anus is now connected 

 by a short canal — the rectum — with the pyloric sac, the junction of the two being not 

 .exactly in the middle of the roof of the sac, but slightly excentric in the direction of 

 interradius V/VI (PI. I. fig. 11). 



The mesenteric relations during these changes require separate description. The 

 rectum grows up between the layers of the epigastric mesentery (p. 33) in interradius 

 V/VI, and has accordingly a sinistral and a dextral wing of mesentery when viewed 

 aborally, the former wing stretching towards radius VI and the latter towards 

 radius V. In the progress of the rectum towards the middle of the aboral surface 

 the sinistral wing of mesentery seems to lead the way, by means of a small thickened 

 part close to the rectum and anus. This part is soon recognisable as a short 

 stout band, containing muscular fibres, which connects the junction between the rectum 

 and the pyloric sac with the central knot of the aboral radial musculature. The 

 muscular fibres of this band are early and definite in their appearance, and the band 

 itself, still more shortened, remains in the adult, where it must play an important part 

 in slinging the rectum and pyloric sac to the aboral body-wall. It may also, in con- 

 junction with the fibrous fold to be next described, serve as a sphincter between the 

 rectum and the pyloric sac, besides aiding in the evacuation of faeces from the 



rectum. 



(In the development oiAsterina, Macbride (15, p. 368) calls attention to a remarkable 

 group of epithelio-muscular cells in the lining of the epigastric coelom on the inner 

 aspect of the body- wall, for which he is at a loss to find a function, unless it be that of 

 o-iving origin to the muscular fibres which dilate the anus. I would venture the 

 suggestion that this group of cells has to do with the formation of suspensory muscular 

 tissue corresponding with the band described above, and perhaps also with the forma- 

 tion of the central knot of the aboral radial musculature.) 



The dextral wing of the original mesentery of the rectum, i. e. that towards radius V, 

 does not shorten so much, but remains in part, and in the adult forms a fold chiefly 

 if not entirely fibrous in character, which stretches from the outer aspect of the 

 junction between the rectum and the pyloric sac to the aboral radial muscle of ray V, 

 joining this about half an inch out from the centre of the disc (8 a). 



