548 



INVEETEBEATA 



CHAP. 



ring of plates. In front of the latter are a group of three very 

 small under-basal plates, whilst in front of these there is a linear 

 series of eleven columnals, each a crescent-shaped ossicle with the 

 concavity directed ventrally. . 



At the conclusion of free-swimming life the larva fixes itselt by its 

 adhesive pit to the substratum, and its anterior portion, surrounded 

 by the columnals, becomes converted into the stalk of the young 

 Crinoid ; but if the free-swimming life lasts longer than one day 

 considerable readjustment of the internal organs takes place before 

 fixation occurs. 



The shift of the right and left posterior coeloms on each other 



Fig. 404. — External views of the embryo and larva of Antedo7i rosacea, showing the 

 mutual relations of stomodaenm and ciliated bands. (After Seeliger.) 



A, embiyo just before hatching, ventral view. B, larva one day after hatching. Letters as in 

 previous figure. In addition, ap, apical plate ; cil.tr, transverse bands of cilia ; Jlx, fixing disc ; siom, 

 larval stomodaeum — the rudiment of the future vestibule. 



increases till the right has become almost completely dorsal and the 

 left ventral, but the latter does not extend ventrally between hydrocoele 

 and ectoderm. In addition to this dorso-ventral shift, a shift in an 

 antero-posterior direction takes place, so that the left coelom shifts 

 posteriorly until it covers the posterior end of the gut, whilst the 

 right coelom shifts anteriorly. If we disregard this second shift and 

 consider the relative positions of the right and left posterior coeloms, 

 when they have just separated from one another, as marking the right 

 and left sides of the embryo, then we should say that the hydrocoele, 

 instead of being mid- ventral, is well up on the left side, and so is the 

 primary madreporic pore ; the larval stomodaeum will also then be 

 an organ of the left side. When the second, the antero-posterior 

 shift takes place, if the description given above has been followed, it 



