Th. Mortensen: On the alleged primitive Ophiuroid, Ophioteresis elegans Bell. 



4-armed specimen, which has been dried ; in the other specimen the skin obscures 

 these piates. This means, accordingiy, that in the very young specimens dorsal 

 piates are formed, more or less regularly, as in Ophiothela. 



The sidearm-plates, as pointed out by Bell, stand out from the sides of 

 the arm, which feature is apparently considered a primitive one, Hke the alleged ab- 

 sence of the ventral piates. (Cf. also Gregory, Op. cit. p. 271). It appears to me 

 very doubtful, whether this position of the side-plates is really a primitive feature ; 

 indeed, I think we shall have to regard their form and position in Ophioteresis 

 as a very specialized feature. This can, however, scarcely be settled at the present 

 state of our knowledge. In any case it is not peculiar to Ophioteresis, as it is also a 

 prominent feature in Ophiothela (comp. textfig. 3); but in Ophioteresis elegans it is 

 especially developed (PI, I. Fig. 10). It appears that in Ophiothela tigris the 

 side-plates stand less out from the arm (PI. I. Fig. 9), and 

 at the same time are apparently somewhat difTerent in form 

 from those of Ophioteresis elegans, which feature, combined with 

 the difference in the shape of the ventral piates described above, 

 would seem to show that O. elegans and tigris may perhaps really 

 be difTerent species. 



Lyman in his description of Ophiothela tigris (p. 11) concludes 

 from the faet that while in one of his specimens the side-plates stand 

 out "independcntly, as padlike ridges", they are in the other spe- 

 cimen "folded close on the arm", that "the animal doubtless has 

 the power of moving the side arm-platcs". I have paid special 

 attention to this point but have failed to find uny such special 

 muscles as would bc necessary to move these piates actively. The 

 position of the piates close to the arm in the one specimen I would 

 supposo to bc due to the preservation, or perhaps, to individuai vari- 

 ation. Observations on living specimens would easily settle the question. 



The arm-spines are, in the outer part of the arm, clegantly formed hooks. 

 (IM. I. Fig. 4). Farther in on the arm they become gradually more complicated, but 

 their original hook-like character is still recognizable (PI. I. Figs 6 — 7). The lower- 

 most spine remains the most hook-like; towards the dorsal side of thr aim they 

 become gradually more simple spines, the upper one — the last devoloping — being 

 Iho simplest. 



The figure of a side arm-plato with two spines represented in PI. I. Fig. 4 

 ro( all« vpry strikingly the so-called pcdi<ellariæ of Trichaster, described by Ludwig*). 



il LuDWio: Trichaster elegans. Morphologische Studien an Echinodermen. I. p. 21H 

 ZuiUchr. r wiss. Zool. XXXI. 1878). 



Fig. 2. „Pedicel- 



laria" from 

 Triduuier degam. 

 (After Ludwig; 

 from GnEGORY). 

 h. hook. s. stulk. 



