Miscellaneous. 191 



theless retain traces of the primitive bilateral symmetry. The exist- 

 ence of this bilateral symmetry is especially evident in those Holo- 

 thurida which creep upon a ventral disk (Psolus) and in the irregular 

 Echinida {Spatangi). There are, in fact, two cases amongst the 

 Echinoderms : sometimes the ventral surface includes an equal part 

 of each ambulacrum, and the mouth is then placed at the centre of 

 the ventral surface (the regular Echinida, Asterida, and Ophiurida) ; 

 sometimes, on the contrary, the mouth is situated at one extremity, 

 and the ventral surface no longer includes a portion of each of the five 

 rays, but only three rays — the other two rays being dorsal (Holo- 

 thurida creeping on the ventral surface). 



In Psolus we find that even the two dorsal rays disappear com- 

 pletely, and there only remain the three ventral ambulacra, of which 

 the median one may sometimes {Psolus squamatus) be almost en- 

 tirely deficient. 



These considerations led Johannes MuUer to distinguish in the 

 five-rayed Echinoderms a trivium and a bivium ; that is to say, two 

 regions, the one including three ambulacra, and the other only two. 



M. Sars remarks that in certain Holothurida (most Cucumarice, 

 Thyone, &e.) we cannot distinguish a trivium and a bivium — all 

 their ambulacra are equal ; but in others we observe slight irregu- 

 larities in the ambulacral areae. Thus in Cucumaria Dicquemarii, 

 Cuv., of the Mediterranean, there are three ambulacra rather more 

 approximated to each other, and each composed of from two to four 

 rows of sucking-feet, whilst the last two ambulacra are rather more 

 distant both from each other and from the former, and are only 

 composed of from two to three rows of feet. Here we have evidently 

 the first indication of a trivium and a bivium ; and from analogy with 

 Psolus we may conclude that the former may be regarded as the 

 ventral, and the latter as the dorsal face. 



This distinction between the belly and the back becomes still more 

 evident in Hemicrepis, Miill. (Phyllophorus, Grube). Here the 

 sucking-feet of the middle of the ventral region are cyUndrical and 

 soft ; on the rest of the ventral surface and on the back they are 

 large, conical, and hardened by numerous calcareous plates. How- 

 ever, as in the genus Holothriria (in which there is also a difference 

 of form between the dorsal and ventral feet), the ambulacra are so 

 greatly developed in breadth that the interambulacral spaces disappear 

 completely, so that it is no longer possible to distinguish a trivium 

 and a bivium. 



In a Holothuride covered entirely with imbricated calcareous 

 scales armed with a spine, for which M. Sars establishes his genus 

 Echinocucumis, there are three complete ambulacra, which, from 

 analogy with Psolus, may be regarded as a ventral trivium, and two 

 incomplete ambulacra, which are dorsal and form the bivium. The 

 tentacles, which are ten in number, equally reveal the bilateral struc- 

 ture of the animal. Two of them, larger than the rest, and furnished 

 with small branches, are in fact lateral (one on each side) ; four 

 others, also furnished with branches, but scarcely so long as the first, 

 are dorsal ; lastly, the other four, which are still shorter and simply 



