150 PHYLUM ECHINODERMATA. 



Enteropneusta, which in the Cephalodiscida furnish the 

 tentacles. The chambers of the third pair both persist and 

 furnish the perivisceral cavities of the adult, but the left be- 

 comes, in accordance with the predominance of the organs of 

 the left side of the larva, much larger than the right, and alone 

 furnishes the germ cells from its lining. The knowledge of these 

 facts we owe mainly to Bury and MacBride. Bury was the first 

 to grasp the importance of the anterior coelom and its trans- 

 ference into the axial sinus, while MacBride went a step further 

 in showing that the coelom was segmented into three chambers 

 on each side, though Metschnikoff * preceded him in having 

 demonstrated the presence of the right hydrocoel in Amphiura 

 squamata. 



In Echinoids (Fig. 99) the single enterocoel divides into two, one on 

 each side. Each of these again divides into two, the hindermost of which 

 lie at the sides of the stomach and constitute the posterior coeloms. The 

 left anterior division develops a water-pore, which is placed on the left 

 side of the dorsal surface, and then becomes constricted into two parts, 

 of which the anterior retains the water-pore and persists into the adult 

 as the madreporitic ampulla and axial sinus, corresponding to the left 

 anterior coelom of Asterids, while the posterior becomes the left hydrocoel. 

 The separation between these two vesicles appears not to become complete, 

 the connecting tube persisting as the stone-canal. The left hydrocoel 

 has at first the form of a disc which is soon transformed into a ring (ap- 

 parently by becoming notched on one side) through which the adult 

 oesophagus later grows. The right anterior division likewise constricts 

 into two parts, but does not develop a water-pore. The anterior of these 

 is the right anterior coelom ; its fate is unknown. The posterior portion 

 is the right hydrocoel, which remains small, never develops lobes, and 

 persists into the adult as the " dorsal sac " which lies beneath the madre- 

 porite. The segmentation of the coelom therefore proceeds in a very 

 similar manner to that of Asterids, and as in them the segmentation of 

 \ the left side precedes that of the right. The two posterior coeloms give 

 rise to the general perivisceral cavity. 



The lantern coelom, which is homologous with the outer perihaemal 

 ring of Asterids, develops as five evaginations of the left posterior coelom. 

 The teeth and jaws are developed from the walls of these pockets and the 

 radial perihaemal canals are outgrowths of them. The genital rachis, 

 genital organs and aboral sinus are developments of the left posterior 

 coelom, exactly as in Asterids. An invagination of ectoderm which 

 becomes closed is formed on the left-hand side of the larva. Its cavity 

 is known as the amniotic cavity, its outer wall becomes thin and is called 

 the amnion, while its inner wall or floor applies itself to the hydrocoel 

 and forms the ambulacral surface of the adult. The epineural canals 

 are developed as invaginations of the floor of this cavity. There is no 

 fixation of the larva during the metamorphosis. Lastly it must be men- 



* Studien iib. d. Ent. d. Echinodermen u. Nemerfcinen, Mem. de VAcad. 

 de St. Petersbourg, 14, 1869. 



