MUSEUM OF COMPARATIVE ZOOLOGY. 49 



Ventrals. — No plates corresponding with the ventrals of the Amphi- 

 ura were found in the young Asterias which were studied. In my 

 account of the development of the plates of Amphiura some difficulty 

 was found in a comparison of the way in which the ventrals develop 

 with the way the " embryonic median row of plates " corresponding with 

 these were formed in Asteracanthion according to A. Agassiz. I was at 

 that time anxious to study the embryonic ventral plates of Asterias, 

 and when opportunity occurred took up the subject for this purpose. 

 I was disappointed, however, for if embryonic ventral plates do exist in 

 some starfishes other genera must be studied. I found no trace of 

 them in any of my young larvae of Asterias.* 



The ventrals of Amphiura are believed not to be represented in 

 Asterias. I have already elsewhere adduced evidence which is thought 

 to be conclusive, as far as mode of formation goes, that the ventrals of 

 Amphiura are not homologous with the embryonic plate of the middle 

 actinal line of the starfish ray as described by Agassiz. My argu- 

 ment then was that the ventrals in Amphiura are unpaired median] 

 calcareous deposits, while the theory would imply that the so called em- 

 bryonic plates of the starfish were formed by a coalescence from two cal- 

 cifications, one on each side (see Agassiz, op. cit., p. 91). As I am unable 

 to recognize in Asterias the middle actinal row of embryonic spines, it 

 is not possible for me to find in Asterias homologues of the ventral 

 plates of Amphiura. 



Interambulacrals. — The interambulacrals of the starfish (Amphiura) 

 are recognized by some authors in the laterals, while others consider that 



* It is, of course, possible that the species of Asterias which I studied may not 

 be the same as tliat in which tlie median actinal "embryonic plates " have been 

 described. The difference in tlie colors of the females, already mentioned, would 

 seem another fact pointing to such a conclusion. 



t This fact is pointed out by Ludwig, and in the light of his studies it is prob- 

 able that the figure of Schultze (fig. 6) represents a ventral in the median line of 

 the under side of each arm. According to Ludwig, the erroneous idea that the 

 ventrals are originally paired structures has been reproduced by Carpenter (Oral 

 and Apical Systems of the Echinoderms, Part II., Journ. Micros. Science, Vol. 

 XIX. p. 21) and Semper (Reisen ini Archipel der Philippen, II. Holothurien, 

 1868, p. 1G2). Ludwig says that, to show tlie homology of the ventral plate of 

 Ophiurans with the adambulacral of the starfish Semper instances the fossil Pro- 

 t'lster Sedgirickii, Forbes, as an Ophiuran with paired ventrals. Semper saw the 

 difficulty of comparison of the unpaired ventral of most Ophiurans with paired 

 adambulacrals of starfishes. It seems to me that Ludwig meets the case of Pro- 

 taster exactly when he says, "Leider ist nun aber Protaster ein nocli so ungenii- 

 jend bekanntes Fossil dass man dasselbe iiberhaupt als Beweismittel in dieser 

 Sache nicht gel ten lassen kann." 



VOL. XVII. — NO. 1. 4 



