DE VEL OPMENT 



3*5 



themselves can, in many cases, grow fresh bodies, and become complete individuals. 

 A star-fish of the genus Linckia commonly avails itself of this faculty ; and it is 

 by no means rare to find big arms with a small body at one end, and four little 

 arms growing out of it ; these are known as comet-forms. This power of repro- 

 duction is probably due to the extension of all the systems of the body into the 

 arms : the arms of brittle-stars, which do not contain all the systems of the body, 

 have not been known to reproduce individuals. In some cases echinoderms have 

 been seen definitely to reproduce themselves by fission or splitting in half. Such 

 division is well known to take place among the sea-cucumbers ; and it is believed 

 by some to take place even in brittle- 

 stars; the specimens of the six- 

 armed green snake-star (Ophiactis), 

 shown in the annexed illustration, 

 being thought to represent the 

 result of such a process. The 

 specimen A consists of two almost 

 similar halves ; but the three arms 

 towards the bottom of the illustra- 

 tion, marked a, are smaller than the 

 others marked a, and indicate that 

 this half is the later grown. The 

 specimen B, which is seen from the 

 back, has only just separated itself 

 from its other half. The separation 

 appears to take place by a forcible 

 though spontaneous rent, and the 

 edges of the wound subsequently 

 grow together, and not merely heal 

 up but reproduce the lost parts of 

 the animal. As a rule, however, 

 echinoderms reproduce by the 

 ordinary sexual methods ; although 

 this, too, presents peculiarities. Just 

 as a butterfly does not develop 

 directly from the egg, but passes 

 through the intermediate larval stage of the caterpillar, out of whose chrysalis the 

 butterfly springs, so the sea-urchin or the star-fish egg gives rise to a larval form, 

 in whose body, as it were, the mature form is developed. The particular shape of 

 the larva varies in the different classes of echinoderms ; but the differences are not 

 essential, and it is clear that all the larval forms are modifications from one primitive 

 type. The changes passed through in the development of the common sea-urchin 

 {.Strongylocentrotus drcebachiensis) are depicted in the illustration on p. 318, in 

 which the drawings are very greatly magnified. 



The fertilised egg divides and subdivides until a round ball of cells is formed. 

 This is then pushed in at one end, as one might push in a soft indiarubber ball, 

 so that there is formed a little sac with a double wall to it (stages 1, 2). Stage 3 



green snake-star (enlarged 5 times). 



