PROF. MACHRIDE ON ECHINODERMS. 1 43 



a curious aspect are with one or two exceptions mere wrinklings or 

 folds of this ciliated band — the organ by which both locomotion and 

 feeding are carried on. Between the gut and the body wall we 

 have on either side a simple sac, part of the future body cavity 

 of the adult ; and one or both of these sacs communicate with the 

 exterior by a pore. 



The remarkable agreement in broad outline, combined with 

 infinite diversity in detail, of these larvae strongly suggests that the 

 characters of a bilateral ancestor of the Echinoderms have supplied 

 the foundation of their structure. Such an ancestor must have 

 been practically on a level with the Coelenterates so far as differ- 

 entiation of the tissues was concerned, but it differed radically from 

 Coelenterates in its mode of life and outer form related thereto. 

 It must have been essentially a swimming animal with a definite 

 head directed forwards. It seems to me to have been a fertile 

 source of error in the earlier days of evolutionary zoology, that, 

 when a living animal of simple structure was found with a con- 

 dition of tissues corresponding to that found in the embryo of a 

 higher form, it was assumed that the ancestor of the higher form 

 resembled in all respects the lower. Thus, for example, because 

 in the general condition of the nervous, muscular and alimentary 

 systems an Actinia recalls conditions met with in the embryos of 

 higher forms, it was supposed that all the higher animals were 

 derived from an Actinia-like ancestor, a conclusion which seems 

 to me to have been unjustifiable when applied to outer form. 



The conclusion that Echinoderms were derived from a bilateral 

 ancestor of very simple organisation is received with increasing 

 favour, and a recapitulation of the grounds which support it would 

 not justify a new paper. The main object of the present com- 

 munication is to endeavour to throw some light on the manner 

 in which this ancestor was converted into the radial prototype of 

 Echinoderms. 



All students of Echinoderm development are familiar with the 

 fact that the first trace of the metamorphosis observable is the 

 appearance of an inner asymmetry. From the body cavity of one 

 side an anterior portion becomes divided off. This becomes de- 

 veloped into a rosette shaped structure, a hoop-like tube with 

 5 outgrowths — the rudiment of the circular canal and the five 

 radial water-vascular canals of the adult. The circular canal is 

 eventually completed by the bending round of the two ends of the 

 hoop till they form a ring. This hoop shaped rudiment has re- 

 ceived the convenient name of hydrocoele. 



It seems to me that the key to the solution of the problem of 

 the ancestry of the Echinoderms lies in the answer to the question 

 "What is the origin and meaning of the hydrocoele?" 



If a developing larva of Asterina gibbosa be carefully ex- 

 amined, it will be found that at one stage both the right and the 

 left body cavity become divided into anterior and posterior parts, 



