PROF. MACRRIDE ON ECHINODEKMS. 1 45 



a well-marked type of structure like that of the Annelid converted 

 it into such a totally different type as the Vertebrate. 



The objection may be made that it follows from this hypothesis 

 that the primitive stock was pelagic, whereas a large part of the 

 modern pelagic fauna can be shown to have been derived from the 

 shore. 



I see no force at all in this objection. If from primitive times 

 the fiercest struggle for existence has taken place in the open sea, 

 it follows that evolution must have gone on with most rapidity 

 there and that animals retaining archaic traits in their organisation 

 could only survive in so far as they became adapted to more 

 sheltered situations. The modern tyrants of the sea are the fish 

 and there is no evidence whatever that their ancestors were ever 

 other than free-swimming pelagic animals. 



It may not be inopportune to remark that such statements as 

 that Vertebrates are descended from Echinoderms, or that Phoronis 

 is a Chordate, are gross caricatures of the view I have sketched 

 above. I much prefer a name such as Protocoelomata for the 

 common ancestors of the three groups mentioned abov^e. 



Having recognized that the hydrocoele is a paired structure, we 

 can see that at any rate one important factor in the metamorphosis 

 is the preponderant growth of the organs of the left side. This is 

 rendered clear by the fact that the right hydrocoele does not 

 disappear, but is found throughout life as a sac embedded in the 

 body wall just under the madreporite. In Asterina gibbosa^ I 

 have been able to trace every st'^ge in the development of this 

 structure from the first inception to its adult condition. Here 

 again I must call attention to the carelessness of Seitaro Goto, who 

 appears to have failed to recognize the same organ in different 

 stages and who speaks of the adult right hydrocoele as a portion 

 of the axial sinus from which it is quite distinct. The axial sinus, 

 the space which in the adult accompanies the stone canal, is formed 

 in Asterids from the union of the two anterior body cavities. 



The fate of the body cavities has not been followed in either 

 Ophiurids or Echinids, but the following facts indicate that when 

 known it will be found to be in substantial agreement with that of 

 Asterids. A sac corresponding to the axial sinus of Asterids, and 

 like it communicating with the stone canal, is found in Ophiurids. 

 where it is known as the Ampulla, and in Echinids, where it is 

 called the cavity of the dorsal organ. A space in adult Ophiurids 

 corresponding to the right hydrocoele, is as we have seen unmis- 

 takeably present in the lar\'a, and was described by me under the 

 designation of sinus b-. In Echinids a similar space has been 

 described by several authors, notably Leipoldt*. In Holothurids 



1 "The development of Asterina gibbosa," by E. W. MacBride. Quart. Joum. Micr. 

 Sc. 1896. 



2 "The development of the genital organs &c. of Amphiura squamata," by E. W. 

 MacHride. Quart. Journ. Micr. Sc. 1892. 



'^ "Das angebliche Excretionsorgan der Seeigel." Zeit. fiir wiss. Zool. 1893. 



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