112 THE VOYAGE OF H.M.S. CHALLENGER. 



ANATOMY OF THE ELASIPODA. 



External Characters. 



The order Elasipoda is distinguished from all hitherto known Holothurioidea by 

 a great number of external characters. In most cases the general appearance makes 

 confusion between the forms of this order and those of the Apoda and the Pedata 

 impossible. The Apoda are either of a strongly-marked worm-like, usually long, narrow, 

 cylindrical shape, or of a fusiform one, with the posterior extremity more or less elongated 

 and strikingly tapered. As an example of the former may be cited Synapta, Eschsch., 

 Chirodota, Eschsch., Haplodactyla mediterranea, Grube, &c, and of the latter Caudina, 

 Stimps., Molpadia, Cuv., Haplodactyla molpadioides, Semp., &c. In addition, the 

 lack of any traces of pedicels, and of any external demarcation between the dorsal and 

 ventral surfaces characterises the order Apoda. In the Pedata the external appearance 

 is characteristic on account of its more or less distinctly traceable bilateral symmetry, 

 but it is to be remembered that also in this order many forms are known especially 

 in the genera Tliyone, Oken, Thyonidium, Dub and Kor., Cucumaria, Blainv., Ocnus, 

 Forbes and Goodsir, &c, in which the body is cylindrical or tapered at each end, or 

 even pentangular, the dorsal and ventral surfaces thus being not clearly marked out. 

 In the Aspidochirotse, especially in the genera Stichopus, Brandt, and Midleria, Yager, 

 but above all in Psoitis, Oken, and Colochirus, Troschel, amongst the Deudrochirotse, 

 the ventral surface becomes flat, sole-like, and most evidently distinct from the highly 

 convex dorsal surface. 



In the Elasipoda the shape of the body is bilaterally syninietrical, the ventral 

 surface being flat or slightly concave, or sometimes insignificantly convex, and 

 as a rule clearly distinguishable from the usually strongly convex dorsal surface. A 

 transverse section of the body (PL XXXVI. fig. 4) generally shows a segment, the 

 arc of which is represented by the back, and the straight line by the ventral surface. 

 The body is in some forms rounded or oval, and in others more or less elongated, 

 thus bearing some resemblance to the Aspidochirotse ; sometimes, however, as, for 

 instance, in the genus Parelpidia, it has the shape of a more or less narrow cylinder, 

 thus becoming almost synapta-like, in which case there is no marked distinction between 

 the dorsal and ventral surfaces, which can only be determined by the position of the 

 processes and the pedicels. Accordingly, the body of the Elasipoda is generally to 

 be considered as psolus-like, the ventral surface being with few exceptions flat and 

 shorter than the dorsal one. Only in the genus Parelpidia do the ventral and dorsal 

 surfaces seem to be of about equal size. Not a single species is found in which the 

 dorsal surface is shorter than the ventral one, as is known to be the case in several 

 of the Dendrochirotee. Sometimes the breadth exceeds the height, and this occurs 

 most conspicuously in the genera Scotoanassa, Euphronides, and Psyclieotrephes, which 



