156 
found this modus of development to occur, e. g. in Cucumaria 
ylacialis, Amphiura vivipara and in some Echinoids, all of them 
being viviparous, with large yolk-laden eggs. (In the viviparous 
Amphipholis squamcita, which has small eggs, the cleavage is 
total and regular; it is then not the viviparity alone that causes 
this peculiar modification of the clevage). Des Arts (Op. cit. p. 12) 
refers to my description of the cleavage stages of Cucumaria gla¬ 
cialis and suggests that „es ist nicht unwahrscheinlich, dass die von 
ihm beschriebenen Falle ebenfalls auf Syncytienbildung zurtickzu- 
fuhren sind“, which must mean that he thinks it an abnormal 
development, analogous to that found by him to occur in eggs of 
Cucumaria frondosa reared under abnormal conditions in aquaria. 
Of course, such an interpretation is entirely out of question. 
The material on which my observations on the development of 
Cucumaria glacialis — as well as that of the other Echinoderms 
mentioned above with a similar modus of development — rests, 
does not proceed from aquaria cultures, but was taken directly from 
specimens found in their natural surroundings. Especially the de¬ 
tailed account of the development of Isomeira vivipara will afford 
the proof that this is the normal modus of development for the 
said forms — but not for Cucumaria frondosa and not for Anle- 
don petasus either. 
VI. On the nervous system of Echinoderm larvæ. 
In my work „Die Echinodermenlarven der Plankton-Expedition“ 
I have figured and mentioned (p. 100. Taf. IX. Fig. 3—4) a regular, 
close series of nuclei lying on each side across the corners of the 
anal area of the larva of Echinocgamus pusillus, from the postoral 
band down towards the place where the band passes from the side 
of the anal area out along the postero-dorsal arm. On account of 
the analogy with the larval nervous system of the Auricularia 
larva, as described especially by Se mon, I suggested that it must 
represent a larval nervous system. As EchinocyamusAarvæ were 
rather plentiful in the Plankton during all the time of my stay at 
the Swedish Zoological Station, I undertook a doser examination 
on the living larvæ of the supposed nervous system. The larvæ 
have the habit of standing very quietly just below the surface of 
