552 INVERTEBKATA CHAP. 



tentacles which belong to one interradius are connected by a 

 common web of ectoderm. 



The gat swells out into a globular form; wandering cells are 

 budded from its wall, pass into its cavity and are absorbed. From 

 its right side a small horizontally directed pouch grows out. This is 

 the rudiment of the intestine which now grows from right to left, 

 lying in the septum dividing the left from the right posterior coelom. 

 The left posterior coelom has been carried back along with the 

 stomodaeum or vestibule, and has now become thoroughly posterior 

 or oral in position, whilst the right posterior coelom is now quite 

 aboral. The main body of the left posterior coelom wedges itself in 

 between stomach and hydrocoele, and it is henceforward known as the 

 oral coelom. Between its two limbs lies the oesophagus. Its right 

 limb also extends inwards beneath the gut so that the opening of the 

 horse-shoe, instead of being directed forwards, is deflected to the left. 

 The left horn of the right posterior coelom wedges itself in between 

 the left posterior coelom arid the gut, and here fuses with the right 

 ventral horn of the left coelom, and the two cavities merge in one 

 another. The right posterior coelom is now termed the aboral coelom. 



On the right side the main body of the right coelom covers the 

 left internally. The two partially coalesced anterior diverticula of 

 the right coelom, by whose opposition the vertical mesentery is 

 formed, extend away forwards and form a central canal surrounded 

 by the five rudiments of the chambered organ. The walls of these 

 rudiments have become thinner and their cavities swollen, especially 

 at their bases, and so the ampullae of the chambered organ are formed. 

 The anterior extension of the anterior coelom has disappeared, and 

 this space becomes reduced to a small rounded sac on the ventral side, 

 opening by the primary madreporic pore (p.c, Fig. 405, B). 



The rudiment of the genital organs appears as the so-called axial 

 organ, which is equivalent, although not exactly homologous, to the 

 genital stolon of other Echinoderms. It appears as a line of 

 enlarged cells on the left side of the longitudinal septum of the right 

 coelom. In the groups so far studied it seems to arise as a prolifera- 

 tion of the wall of the left posterior coelom. This line or cord, 

 becoming detached from the epithelium of the coelomic cavity, sinks 

 into the gelatinous substance of the mesentery and then grows 

 backwards, i.e. towards the calyx or cup ; the hinder portion is in the 

 condition of a thickening of the epithelium, whilst the front part is 

 already sunk inwards. 



The calcareous plates, i.e. the orals and basals, which in the 

 'free-swimming larva formed oblique circles round the body, now form 

 transverse circles ; the orals surrounding the vestibule, the basals the 

 aboral coelom. Five sacculi (Fig. 407) are formed in the wall of the 

 vestibule alternating with the orals. These sacculi are clumps of 

 rnesenchyme cells which become hollowed out and secrete a bright 

 yellow pigment ; they are probably of an excretory nature. 



Next, the oral vestibule becomes opened to the exterior, the 



