6M 



STAKFISHES 



that these become long and tense, and that their 

 sucker-like terminal dues are pressed against the 

 hard surface. There they are fixed, and towardg 

 the attachment the starfish gently Mils itoelf. 

 The protrusion is effected by the internal injection 

 of fluid into the tube-feet ; the fixing is due to the 

 fact that the contained fluid, flowing back again 

 from the tube-feet to the internal reservoirs, pro- 

 duces a vacuum between the ends of the tube-feet 

 and the surface of the rock. 



On the dorsal surface, between the Imses of 

 two of the arms, there is a complex calcareous 

 sieve, somewhat suggestive of the rose of a water- 

 ing-pan. Its pores converge into a 'stone-canal,' 

 which, like a complex calcareous filter, extends 

 vertically through the bodv, and leads into a 

 ring around the mouth. This circiim-oral ring 

 gives off nine transparent vesicles and five radial 

 tubes, one for each arm. Each radial vessel lies 

 in the ventral groove of an arm, roofed by the 

 rafter-like ossicles, and gives off internally reservoir- 

 like bladders or ampullie, and externally the tube- 

 feet. The fluid iu the system seems to pass from 

 the radial vessels into the tube-feet, and from the 

 tube-feet back into the ampullae. It is evident that 

 the water-vascular system is modified for loco- 

 motion, but it is likely that it also helps in respira- 

 tion and perhaps also in excretion. At the end of 

 each arm there is a long unpaired tube-foot which 

 acts like a tactile tentacle. The blood-vascular 

 system is well developed, and in its general course 

 corresponds to the water-vascular system. Respira- 

 tion is in great part discharged by numerous ' skin- 

 gills,' contractile hollow outgrowths from the skin 

 of the dorsal and lateral surfaces. Of excretion in 

 the starfish we know almost nothing. The sexes 

 are separate but quite like one another ; the repro- 

 ductive organs branched like elongated bunches 

 of grapes lie in pairs in each 

 arm ; the genital ducts open 

 dorsally between the bases of 

 the arms. 



The ova are fertilised in the 

 water. It is said that they 

 occasionally begin to develop 

 without fertilisation. The 

 segmentation is complete, a 

 hlastosphere is formed and a 

 typical gastrula The free- 

 swiiiiniing larva, as in other 

 Echinoderms, is remarkably 

 specialised, with two ciliated 

 bands and peculiar arm-like 

 ouifirowths. It is known as a 

 Hipiimariapras a Brochiolaria. 

 The adult is a new formation 

 within the larva, retaining the 

 water-vascular system and mid- 

 gut, but alorbfng or rejecting 

 the provjsional larval structures. There is also in 

 this indirect development a remarkable change 

 from bilateral to radial symmetry. Parental care 

 occasionally occurs among Asteroids ; thus a large 

 Asterias has been seen sheltering it* young within 

 its arms, while in Pteraster and some others there 

 is a dorsal brood-pouch. Many Asteroids break 

 very readily, throwing off their arms when seized. 

 The lost parts are slowly regenerated, and strange 

 shapes, especially the 'comet- form,' often result in 

 the course of regrowth. It is even possible that 

 this 'antotoiny' of members may sometimes be a 

 means of multiplication. 



The commonest European starfishes are species 

 of Asterias or Asteracanthion, Astropecten, Crib- 

 ella, Solaster, Goniaxtcr. In Astropecten and 

 several related forms the food-canal ends blindly ; 

 in Brisinga the arms are very long and arise 

 abruptly from a small central disc, as in Ophiuroids ; 



Fig. 3. Bipinnaria 

 with young Star- 

 fish developing 

 within it 



in Luidia the pedicellarite are three-bladed instead 

 of two-bladed ; and there are many minor differ- 

 ences like the above. There are afxo considerable 

 differences in ex- 

 ternal form, wit- 

 ness the many- 

 rayed Solaster, the 

 pincushion-like 

 Uoniaster, the flat 

 pentagonal Pal- 

 niipes. The largest 

 forms are such as 

 Asterias gigantea, 

 from the Pacific 

 coast of North Fig- 4. Process of budding from 

 America which one of the arms of Linr.Ha multi- 

 measures two feet *>" < After p - d F - Sanurin.) 

 in diameter, and 



Pycnopodia helianthoides, which measures about 

 a yard in diameter, and has over twenty arms. 

 The majority live in coiiiparatively shallow water, 

 but the Ophiuroid-like urisin^a, the widely dis- 

 tributed Hymenaster, the blue Porccllenaster azru- 

 leits, and many others are deep-sea forms. The 

 earliest occurrence of Asteriods is in Silurian 

 strata. 



The Brittle-stars (Ophinroidea) differ from 

 Asteroids in the more centralised body, more 

 sharply defined arms, and more active habit. Com- 

 pared more fully with starfishes, the brittle-stars 

 are more muscular and less limy ; the arms do not 

 contain digestive cieca from the gut nor reproduc- 

 tive organs, and are supported by an axis of limy 

 ' vertebral ossicles ; ' the tube-feet are smaller, 

 apparently tactile and respiratory, and locomotion 

 is effected by the muscular wriggling of the anus ; 

 the groove so well seen on the ventral surface of 

 the starfish arm is here closed in by limy plates ; 

 the alimentary canal ends blindly ; the entrance 

 to the water- vascular system (madreporic plate) 

 is ventral ; the larval form is known as a Pluteus. 

 The popular name 'brittle-star' refers to the 

 extreme ease with which the arms break ; another 

 common name, 'sand-stars,' refers less happily 

 to their occasional occurrence on the shore ; the 

 technical title Ophiuroid describes the snake-like 

 coils of their arms. 



The brittle-stars creep about by wriggling their 

 muscular arms. Some are found creeping on the 



Fig. 6. The Shetland Argus ( Aitrophyton Kutatum). 

 (From Forbes.) 



shores at low-tide, but the majority keep to the 

 floor of more or less deep water. On blocks of 

 coral and the like they swarm in enormous num- 

 bers, twisting themselves most fantastically about 

 the crevices. They are carnivorous animals, feed- 

 ing on small molluscs, crustaceans, worms, and 

 much simpler organisms like Foruimuifera. 



