ZOOLOGY. 



■11 



Leptomedus*, from the velum." It is concluded tliat Limnocodium " will 

 liold a position intermediate between tbe Leptomedusic and the Tracbo- 

 medusiie ; but as tbe greatest systematic importiince must be attached 

 to the structure and origin of the marginal vesicles, its affinity with tbe 

 Leptomedusie must be regarded as the closer of the two." 



ECniXODERMS. 



A NEW ORDER OF HOLOTHURIOID ECHINODERMS.* 



Quite a number of remarkable forms have been procured within the 

 last hve years at great depths of the ocean in various parts of the world, 

 and these have been recently brought together by Hjalmar Theel in one 

 group, differentiated as an " order " closely related to the Holothurioidea. 

 These newly discovered forms essentially resemble the Holothurioids in 

 general structure. It is noted, however, that the Holothurioids are 

 mostly fusiform or cylindrical in shape, although some {e.g.^Psolns and 

 Cuvicrla) are most decidedly bilateral ; the ra*liat« symmetry prevails so 

 far externally that the five rixdial ambulacral vessels and their api>endages 

 are generally similar, or nearly so, and run symmetrically at equal dis- 

 tances from one another from the anal to the apical pole, and they may 

 be even used indiiferently for the purpose of progression ; but, never- 

 theless, of these five ambulacra, three— constituting the trivium— are ven- 

 tral, and the other two — the bivium— are dorsal. While such is the rule 

 however the exceptions are such as to compel us to be cautious, 

 lest we i>lace an undue value on deviations even much less than those 

 recognized as occurring in forms universally conceded to be true repre- 

 sentatives of the order. 



In the newly discovered forms the body is always " distinctly bilat- 

 eral," (1) " the lateral ambulacra of the trivium bearing large, slightly 

 retractile pedicels, disposed either in a single row, or sometimes in two 

 rows, along each side of the ventral surface, and sometimes with another 

 series of larger highly elongated not retractile processes placed exter- 

 nally and above the pedicels ; pedicels of the two lateral ambulacra 

 symmetrically arranged, being more or less distinctly opposed across the 

 ventral surface " ; (2) "the odd ambulacrum naked, or very seldom with 

 a few rudimentary pedicels"; (3) "bivium provided with very long, not 

 retractile processes, often disposed in one or more rows along each of its 

 ambulacra, and more or less distinctly opposed across the dorsal surface, 

 or with only a few rudimentary ones in ii.s anterior part, or with a single 

 very large one resembling a broad, branch e<l or unbranched lobe, and 

 near to it some small papillae;" (4) there are "no respiratory trees;" 

 and (5) the integument is "naked, spiculous, or plated." Such are tbe 

 characteristics common to the forms. While the ordinal value of this 

 type may perhaps be generally disputed, the forms are of no Utile in- 



* Hjalmar Tli6eL The Elasmopoda, a new order of Holotliurideo. 2saiure, vol. xxi, 

 pp. 470-473, Marcli 18, 1880. 



