205 



first stage a typical basket structure, while in their full shape they have 

 the characteristic shape of the larva represented in Joh. Miiller's VII. 

 Abhandl. Taf . V, Figs. 1 — 3, and in the present work, PI. V, Fig. 7, 

 with very long postoral and posterodorsal arms, a posterior transverse rod, 

 very large vibratile lobes but no epaulets. There is then reason to expect 

 that the Diadematid-larvse will prove to represent a distinct, well char- 

 acterized larval "family." 



The Echinothurids, unfortunately, remain entirely unknown as regards 

 their development. While at least some of the deep-sea forms may be ex- 

 pected, on account of the large size of their eggs, to have a direct develop- 

 ment, the small eggs of Asthenosoma varium indicate that this species will 

 have pelagic larvae. It will be exceedingly interesting to see whether its 

 larva will prove to resemble the type of the Diadematid-larvse, as it might 

 perhaps be expected on account of the relation which undoubtedly exists 

 between the Echinothurids and the Diadematids, in spite of the remark- 

 able difference in their test structure. 



Entirely unknown as regards their development are also the Salenidse. 

 Judging from their small genital pores they may be expected to have 

 small eggs and, therefore, probably pelagic larvse, in spite of their being 

 mainly deep-sea forms. It will be most interesting to learn whether their 

 larvae resemble those of their nearest relatives among recent Echinoids, 

 the Arbaciidae. 



Within the family Arbaciidse we now know the larvae of three species 

 of the genus Arhacia, viz. A. lixula, pundulata and stellata, these three 

 being so closely aUke that they are to be distinguished only by very un- 

 important characters. One more larva of the same type is known (PI. VI, 

 Fig. 1), which may perhaps belong to another genus; in any case there 

 can be no doubt that it is also an Arbaciid-larva. The facts thus far made 

 known decidedly indicate that there is a very distinct type of larva 

 belonging to the family of the Arbaciidse, characterized in its first 

 stage by the skeleton forming a basket-structure, in its second stage by 

 the existence of a posterior transverse rod which ends in a pair of long 

 posterolateral arms of the same shape as the other arms. The postoral and 

 posterodorsal rods are of the fenestrated type, but the holes are generally 

 small or entirely lacking in a greater or lesser part of the rod. There are 

 large ventral and dorsal vibratile lobes, but no epaulets. 



It will be of very great interest to learn whether the larvae of the com- 

 paratively few other forms belonging to this very well Umited and un- 

 questionably quite natural family are in conformity with the Arbacia- 

 larvae, as they should be, according to the views of the present author. 

 The polyporous Tetrapygus niger is easily enough accessible for study, and 



