466 



INVEETEBEATA 



CHAP. 



ever, became obliterated. No subsequent observer recorded the exist- 

 ence of this right madreporic pore, although the larvae of Asterias 

 were raised by thousands for experimental purposes by Driesoh, 

 Herbst, and others ; but quite recently Dr. Gemmill has been able to 

 confirm Field's statement. He finds that a right madreporic pore is 

 formed in about one in every ten larvae of Asterias rubens, and in 

 one out of every two larvae of Asterias glacialis. In all cases it very 

 soon closes. 



The appearance of a right and left madreporic pore is the first 



indication of what is really the key 

 to the understanding of Echinoderm 

 development, viz. the fact that the two 

 sides of the larva originally gave rise 

 to precisely similar organs, hut that 

 some of these organs grew and de- 

 veloped on the left side while they 

 atrophied on the right, and that 

 thus an asymmetry was produced. 



The coelomic sacs now begin 

 to grow in length until they form 

 long, narrow, cylindrical cavities, 

 reaching from the prae-oral region 

 of the larva to the posterior end ; 

 and by their form and relations 

 they merit the name " water-tube," 

 bestowed on them by Agassiz 

 (1864). The right and left water- 

 tubes meet one another in the prae- 

 oral lobe and fuse into one (fus, 

 Fig. 358, B), but elsewhere they 

 remain separated from one another by 

 the alimentary canal, and above and 

 below this by a vertical mesentery. 



Now a constriction appears 

 just behind the madreporic pore, 

 which almost, but not quite, divides 

 the left water-tube into anterior 

 On the right side no such constriction is 

 The front division of the left water- 



Fia. 359. — Larva of Asterias 



four days old, viewed from the dorsal 

 surface, showing two madreporic pores. 

 (After Field.) 



Names as in the preceding figure. In 

 addition, l.coe, left coelomic sac ; m.p (left), 

 persistent madreporic pore ; m^jii (right), tran- 

 sitory madreporic pore ; r.coe, right coelomic 

 sac. 



and posterior portions, 

 formed until considerably later, 

 tube may be termed the left anterior coelom, whilst the hinder 

 division is named the left posterior coelom (l.p.c. Fig. 360, A). 

 The posterior portion of the left anterior coelom swells out slightly, 

 and begins to form five lobe -like outgrowths arranged in an open 

 curve. This swelling and its lobes are the rudiment of the water 

 vascular system, and are termed the hydrocoele {hy. Fig. 360). The 

 most dorsally situated lobe is numbered (1), the next (2), and so on. 

 Occasionally a similar five-lobed outgrowth, which we may term the 

 right hydrocoele, is formed as an outgrowth from the right water- 



