31 



preservation being not as good as desirable, it is impossible to ascertain 

 whether there are large pigment spots at the point of the long arms, as 

 is the case in the larvae of A. lixula and pundulata. The vibratile lobes 

 are not yet developed, but there is no reason to doubt that they will be 

 found here as in the other Arbacia-larvse. 



The skeleton (Fig. 5) agrees very closely with that of the A. lixula- 

 larva (comp. J oh. Miiller, VII. Abh., Taf. II, Fig. 10), especially in the 

 noteworthy feature that the postoral rods are fenestrated only in the 

 point. Only the posterior transverse rod differs from that of the A. lixula- 

 larva in being somewhat curved and more robust, more like that of the 

 A. pundulataAarva, which, on the other hand, differs from the present 

 larva and A. lixula in having the postoral rods fenestrated from the base. 



In a plankton sample from off the Azores (36° 13' N. 33° 50' W., 15/III 

 1911), taken onboard the "Ingolf" by Mr. H. Blegvad, was found an 

 Arbaciid larva which is remarkable on account of its exceedingly long 

 postero-lateral arms (PI. VI, Figs. 1 — 2). The shape of the body cannot 

 be made out clearly on account of the rather advanced stage of meta- 

 morphosis, likewise the vibratile lobes cannot be distinguished. The 

 preoral lobe evidently is somewhat altered by the beginning absorption; 

 it has the appearance that the vibratile band passes all round its edge, 

 which cannot be a normal feature in the larva. The structure of the post- 

 oral and posterodorsal rods is like that in the A. lixula larva (both these 

 pairs of arms are broken, so that their full length cannot be ascertained). 

 The posterolateral rods are sparsely thorny, the thorns beginning a little 

 outside the part that is represented in PI. VI, Fig. 2. The posterior 

 transverse rod is slightly curved, with no distinct hole in the middle. 



It seems hardly possible that this larva might belong to A. lixula; 

 the posterior transverse rod is different and even in the more advanced 

 stages of metamorphosis the larva of that species has never been observed 

 to possess such exceedingly long posterolateral arms^). There is then 

 reason to suppose that this larva belongs to one of the other species of 

 the genus Arbacia (A. africana?), or perhaps to Arbaciella elegans. There 

 is also a possibihty that it may belong to a deep-sea form {Coelopleurus 

 or Podocidaris) ; at least such possibility cannot be rejected beforehand, 

 so long as nothing is known regarding the character of the eggs in these 

 deep-sea forms. Anyhow, it has seemed to me worth while calling atten- 

 tion to this conspicuous larval form. 



') Joh. Muller. VII. Abhandlung. Taf. II— III. L. v. Ubisch. Die Anlage und Aus- 

 bildung des Skeletsystems einiger Echiniden, und die Symmetrieverhaltnisse von Larve und 

 Imago. Zeitschr. wiss. Zool. CIV. 1913. Taf. VII. 



