ECHINODERMA GENERAL DESCRIPTION 



" Pelmatozoa "). Setting aside these characters, the origin of which 

 may be traced in individual development, and selecting those 

 common to the early stages of all Echinoderms, zoologists have im- 



agined a phylogenetic 

 stage, the two-sided or 

 Dipleurula stage (Fig. 

 I.), more or less re- 

 peated in the Dipleurula 

 larvae of recent Echino- 

 derms (Fig. II.). The 



M 



Tl 



l.hc: 



animal was marine. Its 



The 



Fio. I. 



Diagrammatic reconstruction of the imagined Dipleurula, long axis Was 



ancestor. Anterior end on left of drawing ; organs of left nnsrprinr anH 



side towards observer, and with stronger outline than those P Ok 



of right side. For description and lettering, see adjoining to the SCa-floor. 



mouth (0) was anterior 



and ventral ; the anus (As) posterior or postero- ventral. The 

 two were joined by an uncoiled gut, perhaps with a stomachal 

 enlargement in the middle. On either side of this lay the coelom, 

 formed by constriction from the larval stomach or archenteron, in 

 other words, an " enterocoel " ; it was divided into a right and left 

 anterior portion (a.c), and a right and left posterior portion (r.p.c 

 and Lp.c). Each anterior vesicle was connected with the exterior 

 by a canal, opening at a dorsal pore (M) on each side the median 

 line, sometimes, perhaps, fusing into one. These canals were in- 

 directly connected (s.c) with posterior offshoots from the anterior 

 coelom, the right and left hydrocoels (r.hc and Lhc). Gonads 

 developed from the coelomic endothelium. The ectodermal 

 epithelium was probably ciliated, and a portion of it in the " pre- 

 oral lobe" (p. I) was differentiated as a sense organ, with longer 

 cilia and underlying nerve-centre (n), from which two gangliated 



FIG. II. 



Dipleurula larvae seen from right side (partly after Lang). 1, Pluteus of Echinoid ; 

 2, Auricularia of Holothurian ; 3, Bipinnaria of Asteroid ; 4, Tornaria of Enteropneustan ; 

 0, mouth ; As, anus ; s, spicule ; np, neural plate with cilia. The strong black lines repre- 

 sent ciliated bands ; the shaded areas show the course of the gut. 



nerves ran back below the ventral surface. In the mesoblastic 

 connective tissue, derived by the migration of cells, there was a 

 tendency to the secretion of crystalline calcium carbonate. Except 



