THE STARFISH SOLASTER ENDECA. 37 



a detailed description of which will be of interest in view of the remarkable manner 

 in which the larval coelomic cavities are disposed. 



Sefaration of Enteron from Anterior and Posterior Coelom (PI. III. figs. 25-33). — As 

 the gastrula elongates, the cells lining the aixhenteric cavity become diminished in size, 

 partly owing to the fact that they are now spread over a larger area, but chiefly because 

 a considerable amount of mesenchyme has already been formed at their expense. 

 Even in the early cone stage the diminution in size of these cells may be seen to be 

 unequal in amount in different places, a zone of larger cells being left rather less than 

 halfway up the archenteric wall. For purposes of description we may divide the 

 zone into two bands, an anterior marked A A in figs. 25-27 and a posterior marked B B ; 

 and it may be stated here that the anterior band contains the tissue which gives origin 

 to the right and left lateral diverticula of the anterior coelom, while the posterior will 

 form the walls of the middle chamber or enteron. Even at this early stage flattening 

 from side to side of the posterior part of the elongating gastrular cone is beginning to 

 take place. Usually, also, the base of the cone is slightly bevelled, in such a way that 

 a rather acuter angle is formed on its postero-ventral than on its postero-dorsal side. 

 With the help of these indications it is often possible to recognise the difi'erent aspects 

 of the larva from a very early stage. By turning over the cleared larvae under the 

 microscope one can make out (1) that the whole thickened zone (including both 

 A and B bands) is more definite at the sides than dorsally ; (2) that the want of 

 definiteness seems best marked a little to the right of the mid-dorsal line ; (3) that 

 there is also some slight deficiency of the zone towards the mid-ventral line. 



At a somewdiat later stage, when the length of the cone is double its breadth and 

 the ventral dimple is appearing, the following further points may be made out : — 



(1) the posterior part of the larva ir more distinctly flattened from side to side ; 



(2) the band marked B B of the archenteric wall is projecting inwards at the sides 

 but not dorsally or ventrally, thus forming two ridges within the archenteron which 

 may be called respectively the right and left enteric folds ; (3) dorsally the gap 

 between these folds lies a little to the right of the middle line ; (4) ventrally the 

 corresponding junction lies a little to the left of the middle line; (5) in optical section 

 the right fold seems to be rather larger than the left, though the apparent size varies 

 somewhat according to the side from which they are seen, and examination of serial 

 sections is required to bring out the diiferences with exactness. The latter method 

 shows that towards their dorsal junction the right fold is distinctly the larger, though 

 the less prominent. Ventrally the opposite condition is found, though here the 

 differences are less. Near the middle the right fold has still a slight predominance 

 over the left. 



The archenteron now shows anterior and posterior chambers with an isthmus 

 between. The isthmus is short from before backwards, narrow from side to side, but 

 elongated dorso-ventrally. The right and left enteric folds forming its opposite sides 



