32 DE. J. F. GEMMILL ON THE DEVELOPMENT OF 



(2) Posterior Coelom and its Derivatives. 



The posterior coelom is at first hardly to be distinguished from the middle chamber 

 or enteron. The detailed account of its mode of separation is given on p. 37. It 

 soon begins to extend in the sagittal plane, giving rise to dorsal and ventral horns. 

 The ventral horn grows with slightly gi-eater rapidity than the dorsal one, and partly 

 for this reason the opening of this coelom into the enteron undergoes some degree of 

 ventral displacement. 



The two horns continue to grow forward, insinuating themselves between the body- 

 wall and the gut, and keeping still in the sagittal plane, except that the ventral one, 

 which is also the longer, swerves a little to the right side of the middle line. 



The dorsal horn extends forward as far as the dorso-anterior angle of the larval 

 body, where it is brought to a stop meantime by abutting against the central part of 

 the anterior coelom, into which the hydroporic canal opens. The ventral horn on its 

 part grows forward as far as it can, and then bends dorsalwards in front of the enteron 

 in the direction of the hydropore. 



During the early stages of metamorphosis, the growth of the ventral horn, aided by 

 the flexion and torsion of the preoral lobe, plays a chief part in separating the right 

 larval (or epigastric) and the axial regions of the anterior coelom from one another, 

 and also in obliterating the stalk of the preoral cceloni. In doing this the tip of the 

 ventral horn may be described as bifurcating temporarily into right and left processes. 

 The right process is the smaller, and abvits against the neck of the right lateral 

 diverticulum, which it helps to occlude (p. 27). It then disappears as a distinct 

 process by merging with the main cavity of the ventral horn. 



The other process of the tip of the ventral horn swerves to the left or oral side, 

 expanding as it does so. This brings it against the stalk between the preoral and 

 axial coelom, which it helps to occlude. After occlusion is effected it extends across 

 the position of the larval neck towards the madreporic interradius, excavating the 

 tissue in front of it and reaching its final position over the first radial pouch of 

 the hydrocoele. It now ceases to be recognizable as a distinct process by merging with 

 the general cavity of the ventral horn. 



The manner in which these changes occur and the part played by the ventral horn 

 will be better understood if one refers to what was said on page 22 about the 

 adjustment of the disc-arm rudiments to the radial pouches of the hydrocoele. The 

 tip of the ventral horn of the posterior coelom is contained within the (larval) ventral 

 lip of the nuchal notch on the disc. This lip or lobe has to travel across the region 

 of the neck in order to reach its final position over pouch I of the hydrocoele. 



The walls of the dorsal and ventral horns now approach back to back in the madre- 

 poric interradius to form a thickened mesentery or septum stretching from the enteron 

 outwards to the body-wall. Within this septum are contained the structures forming 

 the axial complex, the development of which has already been described (pp. 29-31). 



