98 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



COMPARISON OF THE LARVA OF ANTEDON ROSACEA WITH THAT OF OPHIURA 



BREVISPINA. 



While I was studying the larva of Ophiura brevispina, characters were constantly being 

 found which reminded me of the larva of Antedon as described by Bury (1). Some of these 

 points of resemblance are no doubt oidy superficial, but others are such as to make it worth the 

 while to devote a short chapter to the similarities of the two larva'. 



The entire ciliation of the very young larvae gives place in both to a series of transverse 

 ciliated bands, five in Antedon, four in Ophiura. The band nearest the anterior end of the Ante- 

 don larva, however, is small and incomplete. Two bands only in eacb case surround that part of 

 the larva from which the disk is formed. 



The blastopore in both larva*, after shifting from a posterior position to one on the ventral 

 surface, closes and the archenteron loses its connection with the ectoderm and lies free in the body 

 cavity. 



In the seven-day embryo of Antedon and Stage "C" of Ophiura the hydrocoele is a horseshoe- 

 shaped structure lying in the posterior ventral part of the larva' with the open end directed 

 anteriorly, and in each case the plane of radial symmetry of the hydrocoele is at right angles to 

 the plane of bilateral symmetry of the larvte. 



In the formation of the paired tentacles from the radial water canals the process is begun 

 centripetally in both larva?, the second pair of tentacles appearing between the first pair and the 

 water ring instead of between the first pair and the end tentacle, as is the case in the other groups 

 of echiuoderms. 



In the five-day Antedon larva and those stages represented by "D" to "F" in Ophiura the 

 stalk and larval organ are strikingly similar, both in shape and position, the two structures being 

 anteriorly directed in swimming. 



The stem of the Antedon larva, as a result of metamorphosis, comes to be an aboral structure, 

 and just before the disappearance of the larval organ from the opbiuran larva it is to be found as 

 a small knob, not in the center of the aboral disk, it is true, but on its edge. In the starfishes it 

 may be recalled that the preoral lobe disappears on the oral surface of the metamorphosing star. 



To the above larval characteristics may be added the similarity which exists in the disposition 

 of the alimentary and coelomic systems in the adult forms. 



In both Crinoids and Ophiurans the digestive apparatus is confined to the disk. 



The body cavity is continued into and to the ends of the arms. When a transverse section of 

 a pinnule of Antedon is compared with a transverse section of an ophiurid arm, the following 

 striking correspondence is found in the parts: Aborally, segmentally arranged calcarious ossicles 

 and muscles are present; a continuation of the body ccelom runs between and oral to the muscles; 

 connected with and on each side of this central brachial body cavity are two other cavities, the 

 subtentacular canals of Antedon. These latter in the Ophiurans are connected with perilnemal 

 space in each vertebral segment. 



The radial water tube lies between the subtentacular canals, and in each segment sends out a 

 pair of tentacles. The tentacles in both the Crinoids and Ophiurids are devoid of the terminal 

 suckers, which are so characteristic of the other echiuoderms. 



Separating the radial water tube from the nerve cord is to be found the radial perilnemal 

 sinus. 



In ophiurans a subneural space is present which is not represented in the crinoid arm. This 

 is due to the fact that in Antedon the nervous system is superficial, while in Ophiura it has been 

 invaginated, and with its invagination a space has also been carried in below it. 



LITERATURE CITED. 



1. Bury, H. "Early Stages in the Development of Antedon rosacea." Philosophical Transactions, vol. 170. 1888. 



2. Bury, H. "Studies in the Embryology of the Echiuoderms." Q. J. Mie. So., vol. 29. 



3. Bury, H. "The Metamorphosis of Echiuoderms." Q. J. Mic. Sc, vol. 38. 



4. Cuenot, L. " Etudes Morphologiques stir les Echinodermes." Archiv. Biol. t. XI. 



5. Goto, S. "The Metamorphosis of Asterias pallida, with special reference to the fate of the Body Cavities." 

 Journal of the College of Science Imp. Univ. Tokyo, .Tapan. Vol. X, Pt. III. 18!i*. 



6. Grave, C. " Notes ou the Development of Ophiura olivacea." Zool. Anzeig. Bd. XXII. No. 580. 1899. 



