EMBEYOLOGY OF ASTERIAS. 185 



organ formed only changes of substance have taken place ; but now 

 little swellings appear in five points on the sides, and the spherical 

 portion of the germ becomes flattened by lateral dilatation. 



(485.) The minute animal has grown to a more hemispherical shape ; 

 and from this time there is an upper and a lower surface to its um- 

 brella-like disk, and a tubular part and a swollen portion to the 

 peduncle. As soon as the peripheric part of the disk begins to 

 spread, five small tubercles may be observed forming underneath ; and 

 into these tubercles the peculiar aspect of the middle one extends. 

 Soon other prominent swellings appear, two to each of the former ones, 

 and subsequently two more. While this is going on, calcareous nets 

 are formed by the accumulation of crystals in the cells of the germ. 

 At first there are simply isolated crystals, formed as nuclei in the cells ; 

 then several close together will unite and form a little irregular mass ; 

 and they will at last combine, so as to constitute a network of solid 

 substance, arranged very regularly, and gradually becoming more and 

 more numerous, marking out more and more distinctly the rays of the 

 embryo Star-fish. The tubercles of the lower surface, growing more 

 prominent and elongated, are finally transformed into the suckers, or 

 ambulacral tubes. With the addition of new calcareous nets, these latter 

 become more numerous, and form, finally, rows of tentacles. Other 

 changes have also taken place. The cells within the peduncle have under- 

 gone alteration : some have become moveable, and a kind of circulation 

 is going on in them. The internal space along each ray has become 

 more transparent ; the ambulacral tubes have become hollow ; and from 

 that time there seems to be a communication between the external 

 water and the internal structure. "What remains of the yelk is more 

 distinctly circumscribed in the centre of the animal, extending as a 

 star-shaped disk into the rays. The radial portion becomes, finally, 

 distinct from the central one ; and we have at last an internal cavity, 

 which is the stomach, from which the cgecal appendages of the rays, 

 with their liver-like organ, will be developed. 



(486.) The peduncle is reduced to a mere vesicle ; a hole is formed 

 in the centre of the lower surface, constituting the mouth ; around this 

 a circular thread becomes visible, answering to the nervous system, 

 from which other threads extend towards the extremity of the rays ; 

 and by the time the young Star-fish has attained the size of about a 

 line in diameter, it has thus assumed the form and structure of a per- 

 fect animal. 



(487.) Among the most interesting contributions to our knowledge 

 of this group are the researches of Professor Miiller* relative to the 

 embryonic condition of the Opliiuridce, from which it has been ascer- 

 tained that, during the progress from the egg to the mature condition, 



* Ueber die Larven und Metamorphose der Ophiuren imd Seeigel. Berlin 

 Trans. 1846. 



