762 HOMOLOGIES AND AFFINITIES OF ECHINI. 



find in the corresponding stages of Echinoderras. The mode of formation 

 of the ambulacra! tubes in Ctenophorae is identical with that of the water- 

 system of Echinoderms ; they are formed as diverticula from the digestive 

 cavity exactly as the water-system of Echinoderms is formed as a diverticu- 

 lum from the digestive cavity. As 1 propose to return to this subject in 

 a detailed Embryology of our common Ctenophorae, I only allude to this 

 important structural affinity to maintain the views 1 still hold of the close 

 relationship of the Ctenophorae and other Acalephs and Polyps with the 

 Echinoderms. 



It, seems scarcely necessary to criticise the views lately thrown out by 

 Haeckel of the composite nature of Echinoderms, in which he goes so far 

 as to suggest the possibility of each arm of a Starfish and Ophiuran being an 

 individual. This view is so contrary to all our ideas of the homologies of 

 these animals with Sea-Urchins and Ilolothurians, that, unsupported as it is 

 by any data, and simply thrown out as a theoretical hint, it need not detain 

 us any longer. The hypothetical genealogical tree made by Haeckel of the 

 derivation of Echinoderms is entirely at variance with what we know of the 

 embryology and of the geological succession of the class, and no positive proof 

 can as yet be given of any other affinity between Worms and Echinoderms 

 than the superficial resemblance of some embryos of Worms and Echino- 

 derms. In his genealogical tret' of the Echini, Haeckel has been compelled 

 to derive from the Cassidulidae, the ( 'lypeast roids. as well as the Spatan- 

 goids, which culminate in the present period in two equivalent groups. 

 This we can explain in no other way except that the Clypeastroids were a 

 retrogressive group, and yet the variety of the forms under which they ap- 

 pear (lining the Tertiary period shows anything rather than the dying out 

 of the suborder. He leaves entirely unexplained (not even questioned) the 

 sudden passage from Echinocidaridae to Dysasteridae and to Galeritidae. We 

 might understand, as stated before, the passage of the Echinocidaridae to the 

 Galeritidae and to the Clypeastridae both from anatomical and embryological 

 data, but such an anomaly as the transition or genetic connection of the 

 Echinocidaridae with the Dysasteridae and Cassidulidae, and hence with 

 Spatangoids, is the merest hypothesis, entirely unsupported either by ana- 

 tomical or embryological data. 



Cambridge : Electrotypes! and Printed by Welch. Bigelow, & Co. 



