10 



ECH1NODERMA- GENERAL DESCRIPTION 



not merely pulled it up, but pressed it into a horseshoe curve, 

 with the opening directed to the anal side. The left posterior 

 coelom was curved in like manner. The further elongation of the 

 fixed aboral end involved lobes of the right posterior coelom 

 and initiated their downward extension, first on the right side of 

 the anterior coelom, then gradually curving round it. This torsion 

 and shifting of internal organs may be compared with the simpler 

 case of streptoneurous Gastropoda. 



Careful study of the two diagrams, representing the Dipleurula 

 (Fig. I.), and the primitive Pelmatozoan or fixed stage (Fig. VII), 



ax 



Fio. VIII. 



Sections of Antedon larva, semi-diagrammatic (1-4 after Seeliger, x85). (5, 6 after Bury). 

 1, longitudinal section with preoral lobe turned downwards, the reverse of Fig. IV. ; the anterior 

 coelom extends into this, as well as upwards into the parietal canal. 2, transverse section in 

 the neighbourhood of the hydropore, showing the parietal canal leading to it ; two lobes of the 

 hydrocoel and the lower end of the stomach are seen to be at the same level as right posterior 

 coelom and lower end of vestibule. 3, part of a longitudinal section in the same direction as 

 Fig. 1, but passing through the hydropore and the five primitive lobes of the hydrocoel not 

 yet connected therewith ; between the two is a small extension of left posterior coelom ; adjoining 

 sections show that the hydrocoel is still horseshoe-shaped, but will eventually close along the 

 gap between lobes 1 and 5 to form the hydrocircns. 4, transverse section at a slightly more 

 orad level than in 2, and in an older larva; the vestibule is here closed over; note the 

 mesenteries between right and left posterior coeloma. 5, transverse section of an older larva, 

 in which the vestibule has passed right up to the oral pole, and a rectum (r) has formed ; 

 between this and the stomach is seen the rudiment of the axial organ ; the parietal canal 

 remains, but the stone canal now opens into it (x!25). 6, median longitudinal section, the 

 vestibule still open, columnals forming around extensions of right posterior and anterior 

 coeloms ; left posterior coelom is seen above the stomach as a lobe of the hydrocoel (x!20). 



Explanation of letters ac, anterior coelom ; ax, axial organ ; cc, ciliated bands ; col, 

 columnals ; he, hydrocoel ; l.pc, left posterior coelom ; nn, nerves ; p, hydropore ; par, parietal 

 canal ; r, rectum ; r.pc, right posterior coelom ; st, stomach ; st.c, stone canal ; v, vestibule. 



will enable the student to appreciate tne peculiar position of the 

 internal structures in the Antedon larva, of which a few sections 

 are here given for comparison (Fig. VIII.). The structure is far 

 more complicated than in Fig. VII., owing to extensions from 

 the coelomic cavities. In the earlier sections the hydrocoel is still 



