PROF. MACBRIDE ON ECHINODERMS. 1 47 



larva by which it is easy to distinguish the Asterid larvae from 

 those of Ophiurids, Echinids and Holothurids. 



If we suppose a creeping and then a fixed condition to have 

 intervened between the ancestral condition represented by the 

 larva and the modern condition, we can find at least a suggestion 

 as to the cause of the loss of bilateral symmetry. It is too often 

 hastily assumed that bilateral symmetry is something fundamental 

 in the organisation of animals ; or again that it is one of two 

 equally probable alternatives, viz. symmetry and asymmetry, where- 

 as bilateral symmetry is really only one out of an innumerable 

 number of possible alternatives. As Roux has justly pointed out 

 the wonder is not that animals should be asymmetrical but that 

 the}' should be symmetrical, and the persistent bilateral symmetry 

 can be best explained by pointing to its functional importance for 

 free-swimming animals which pursue a definite direction. It 

 is significant that one of the first signs of the unhealthiness of an 

 Echinoderm larva is the absorption of some of the arms of one 

 side, thus producing marked asymmetry. 



The moment that a fixed stage is acquired bilateral symmetry 

 ceases to be of importance, whereas a radial arrangement of the 

 external organs is of advantage. Any tendency to inequality 

 in the hydrocoeles would be taken advantage of to produce this 

 result, and the predominance of the left hydrocoele need no more 

 surprise us than the existence of one genital duct in Cephalopods 

 or of one ovary in the fowl. 



To some one result of the above considerations will be unwel- 

 come, namely that the Echinoderm stem is cloven into two at the 

 ver}- base, and that from the inception of the fixed stage on, the 

 Crinoids have pursued a different path to that of other Echinoderms. 



I can only say that the embryological evidence appears to me 

 to be quite unequivocal on this point, and that the utter hopeless- 

 ness of attempting (even from the stand-point of comparative 

 anatomy) to homologize the aboral poles of Asterids and Crinoids 

 has already impressed itself on competent palaeontologists such as 

 E. Bather. 



An explanation of this extraordinary difference may perhaps, 

 like that of many other zoological divergences, be found in the fact 

 that the Asterid and Crinoid stocks have adopted two different 

 plans of meeting the same difficulty. The study of the develop- 

 ment of sessile forms such as Cirripedes and Ascidians seems to 

 show that one of the first problems which a newly fixed form had 

 to contend with, was that of bringing the mouth into a favourable 

 position for catching food. Like the Cirripede and Ascidian the 

 Crinoid rotates the mouth backwards and upwards; the Asterid on 

 the other hand rotates it on to the left side whilst a distinct ventral 

 flexure of the disc on the stalk takes place. In all cases of develop- 

 ment where the larval mouth is continuously transformed into the 

 adult one a movement to the left takes place. 



lO — 2 



