Fewkes.] 
110 
[Dec. 5, 
events their predominance in size and their early appearance would 
seem to invite us to regard them as of some importance in the mor- 
phology of the echinoderms, and they possibly tell some story of 
the phylogenetic history of the groups to which the genera belong. 
It is an interesting and significant fact that in the genus Atnphiura, 
an opliiuran which does not have a free pluteus stage, but in which 
the young are carried in brood sacs until they are quite well ad- 
vanced, these spines are not developed. Unfortunately, the young 
of the starfish, Pteraster, which also carries the young in similar 
but not homologous sacs, is so little known that we are unable to 
say whether these spines are present or not, and the same is true of 
the so-called viviparous sea-urchin, Hemiaster. We cannot state ‘ 
whether these large early formed spines are limited to genera where 
the young stages are nomadic, and have plutean or brachiolarian 
stages. Nor has the physiological function of these appendages 
been discovered. It is probable that the uses which they perform 
may be different in different genera. 
The physiological role of the flattened primary spines of Aste- 
rias is probably not the same as that of the hook-shaped primary 
spines of a genus like the opliiuran, Ophiocoma. In the former case 
we are tempted to regard them as connected with locomotion and 
possibly as survivals of similar structures in an ancestral form where 
they played an important part in the swimming of the animal. A 
somewhat similar function of modified spines is to be found in the 
“fins” of a genus of ophiurans, Ophiopteron, lately described by 
Ludwig . 1 It seems not impossible that in an ancestral form of Aste- 
rias now unknown, there existed organs not unlike those preserved 
in Ophiopteron which served as flappers by which the starfish was 
propelled through the water. I have carefully watch'ed the young 
starfish in its varied movements but have never seen any flapping 
of these paddle-like spines in this genus. It is possible that the 
highly developed brachiolaria has vicariously performed the func- 
tion of these organs so that in Asterias at least they are function- 
ally useless although they still preserve somewhat the fan-shaped 
form of the ancestral organs where they may have served as flap- 
pers for propulsion of the animal. 
In connection with the possibility that the flat, fan-shaped spines 
of Asterias may be in some way connected with propulsion one 
*Zei£ f. wiss. Zool., Vol. xlvii. 
