130 Echinoderma. 



five arms are present^ one is produced aJong each ; whilst, if there 

 are ten^ each groove forks just as do the arms, and sends a small 

 furrow tO' each pinnule. Along each edge of the furrows both 

 of 'the arms and of the pinnules there is a row of small, soft feet or 

 suckers (so-called tentacles) ; a water vessel runs below it, and gives off 

 branches to the feet. (For the stone-canal, seep. 124). The 

 genitalia, which are similar in both sexes, extend as long tubes through 

 the arms' and give off branches to the pinnules ; these branches alone 

 produce ripe reproductive cells, whilst the main trunk is sterile : the 

 ova and spermatozoa escape by small openings in the pinnules, the 

 latter having become much swollen by the ripe genital products. 



Development is known only for Antedon, which as an adult 

 has no stalk. The ovate body of the newly-hatched larva is provided 

 with four ciliated girdles and with a tuft of cilia at the hinder pole. 

 After swimming freely for sonae time it attaches itself by one end, which 

 elongates, to form a stalk, whilst the arms bud out at the other pole. 

 Later on, the body with the arms breaks from the stalk, and the animal 

 is free-swimming for the rest of its life. 



The stalked Crinoids are almost exclusively abysmal, whilst those 

 which are free-swimming usually occur in shallow water near the 

 coast. Crinoids feed upon microscopic organisms which are driven to 

 the mouth by ciliary movements in the furrows already mentioned. In 

 earlier geological periods, especially in the Silurian and Cretaceous, 

 they were as common as they ' are now scarce, and were principally 

 represented by stalked, forms, in fact entirely so down to the Jurassic. 

 Genera, species, and individuals, were all abundant. 



The following may be mentioned as examples of present-day 

 forms :^- 



1. Bhizoorinus lofotensis, a small, long-stalked (to 8-o/m) form, with five 

 (occasionally 4, 6, or 7) simple arms ; tte end of the stalk is provided with 

 branching root-like cin-hi, by which it attaches itself to objects at the bottom 

 of the sea, whilst elsewhere there are none. The animal was first met with off 

 the Lofoden Islands, at depths of 100 — 300 fathoms, but afterwards, also at great 

 depths in various other localities. 



2. Sea Palms (Pentacrinus) are lai-ge animals with ten arms, which may 

 divide repeatedly ; the strong stalk is beset with whorls of jointed cirrhi down its 

 whole length. At great depths in warm seas. 



3. Antedon or Comatula, a stalk-less Crinoid, with ten or more arms. In 

 the young stalked condition, ciiThi occur only at the junction of the stalk and the 

 body ; these cirrhi persist after the animal has broken away, and by means of 

 them, Antedon can climb upon various foreign objects, whilst it can swim with 

 its arms. A. rosacea occurs in the Mediten-anean and the Atlantic. 



Class 2. Asteroidea. 



In this class the body is always discoid (the principal axis 

 short) and drawn out into a number of arms (usually five), for the 

 radii are better developed than the inter-radii. The tube-feet 



