ASTEROIDEA 



47 



Key to the larvce of Asteroids {Bipinnaria^). (Fig. 28.) 



1. Anterior part of body much prolonged . , . Luidia 



not „ .... 2 



2. The fully formed larva a Brachiolaria .... 3 



,, ,, ,, not a Brachiolaria . . Astropecteti 



3. Papillae in a whorl round the point of the Brachiolaria-arms 



Asterias 

 Papillae in a series along the sides of the Brachiolaria-arms 



Porania 



The sea-stars are slowly moving animals, which proceed with 

 a sliding movement over the bottom, progressing on the points 

 of the tube -feet .2 The sucking disks are generally not used when 

 w^alking on smooth bottom, but only when climbing. Most of 

 them are very voracious, eating all sorts of animals, especially 

 echinoderms, molluscs, and worms. They are able to swallow 

 relatively large animals, the mouth 

 being ^ery extensible ; but even 

 such animals as are too large to be 

 swallowed — e.g. fishes — they are 

 able to devour, by protruding the 

 stomach over them, thus digesting 

 them outside their body. Mussels 

 also which are too large to be 

 swallowed they are able to manage. 

 The sea-star then assumes a char- 

 acteristic position above the mussel, 

 joining the arms round it, Avhile 

 -raising its back (Fig. 29). Through 

 the steady, uniform pull of the 

 tube-feet the mussel is forced to open 

 its valves ; the sea-star then protrudes its stomach and the mussel, 

 which is probably paralysed by the juice of the sea-star*s stomach, 

 is thus devoured. The sea -stars in this way do great harm 

 to oyster cultures. Fishes also are attacked by sea-stars, but, 

 of course, mainly such as are hanging in nets and thus unable 

 to escape. Some few sea-stars, e.g. Ctenodiscus, simply swallow 



^ Some other species of Bipinnaria have been described in the author's 

 work. Die Echinochnnenlarven d. Plankton- Expedition, 1898. Possibly 

 some of these will prove to belong to sea-stars which occur also in British 

 seas, but for the present nothing more definite can be said, and it must 

 suffice to refer to that work. 



^ A tropical sea-star, Luidia clathrata (say) ; it is stated to be able to 

 swim by means of its tube -feet. 



Fig. 29. — Sea-star {Asterias 

 ruhens) in the act of devour- 

 ing a mussel. (After H. 

 Blegvad ; from Danmark' s 

 Fauna.) 



