550 



INVERTEBRATA 



CHAP. 



over to the right posteriorly, that it becomes horse-shoe shaped with 

 the concavity of the horse-shoe directed forwards. 



If the free -swimming stage lasts long the rudiment of the 

 oesophagus appears as a solid peg-like outgrowth of the stomach, 

 which grows between the two limbs of the hydrocoele towards the 

 floor of the stomodaeum ; and the rudiment of the stone-canal also 

 appears as an outgrowth from one end of the hoop-shaped hydrocoele. 

 According to Seeliger the original connection between anterior coelom 



Stx 



al 



FIG. 406. Longitudinal sections through free-swimming larvae of Antedon rosacea. 



(After Seeliger.) 



A, section in the sagittal direction, but not in the sagittal plane, through a larva which has just 

 been hatched. B, median sagittal section through a larva which has been hatched for twenty-eight 

 hours and is about to fix itself. Letters as in two previous figures. In addition, ax.c, one of the axial 

 prolongations of the right posterior coelom ; ch.o, rudiments of the chambered organ outgrowths of the 

 right posterior coelom ; l.neri', fibres of the larval nervous system. 



and hydrocoele was in the middle of the hoop, as in Ophiura brevis, 

 and for that matter in Asterina gibbosa also. The inesenchyme in 

 the anterior portion of the larva, towards the centre, consists of 

 closely packed rounded cells, and in these the calcareous columnals 

 are formed, but towards the periphery the mesenchyme forms 

 elongated fibre-like cells, which later form muscles and fibres in the 

 stalk of the young Crinoid. Each lobe of the hydrocoele develops a 

 pair of lateral lobes, and the original five lobes begin to protrude into 

 the stomodaeum as free tentacles (Fig. 405, B). 



Then the larva, having found a suitable spot, fixes itself to the 



