Embryology of Echinoderms

EM B R Y O L O G Y O F ECH I N O D ER M S . B Y A L EX A ND ER A GA S S Z . ‘ I M HE M I ICA A DEMY V L. IX . FRO T ME O RS OF THE A MER N A C , O n 2d 1 864 . Comm u icated February , P ub lishe d Alarch , 1 864 . M EM O I R S O F TH E A M ER I C A N A C A D EM Y . On th e Embryology of Ech inoderms . B Y A L EXA N D ER A GA SSIZ . Co mm un icated Fe b u a 2d 1 8 64. r ry , THE following account of the embryology of a few of our Echinoderms , though by s o f no means complete , will fill everal the gaps hitherto existing in the knowledge of " th e development o f Echinoderms ; and in the h Ope that something may be added to o u r s n o f o f o f under tandi g the general plan development Echinoderms , it is here given n o s h o w is i its present c ndition , in order to how far the plan of development identical ’ f I s l ih . O h i the di ferent orders of Echinoderms ha l take up in turn the Echinoids , p s ff a uran , and Holothurians , and then compare these di erent larv e with what we know o f o u r s se e h o w f already of the development common Starfi h , to close the agreement o o f o f n is s s the mode formation the you g Echinoderm in these four orders , and to ati fy s o n s " ourselves how true are the uggestions made at that time , a very uperficial ac o f A ste racan th io n quaintance with any other Echinoderm larva except that , concerning f f the function of many o the organs o these wonderful larvae . E E B TOX OPN UST S D RO A CHIEN SIS A G . T h e larvae o f our common Echinus resemble most clo sely,in some of their earlier tes livi us A . o n To m neus . VII . stages , those of p d g , figured by Muller Plates VI and of * M lle r w w his fourth Memoir . The figures which ii gives correspond ith hat I have ‘ i n ic E h in U E J . b e r Larv e i u d d s de r c o dcrm n . h Uc c A b amll n . M LL R , die Metamorpho e Vierte u g Be r 1 85 2 . lin , VOL. IX . 2 ON TH E m rs ar o no o r o r E CH IN O D ER M S . observed in larva obtained by artificial fecundation . He succeeded in tracing them is as as . for about three weeks , which not quite long I have kept them alive Muller h as n o t o f s s unfortunately given us any figures the very first stages of thi specie , nor h as he found the adult larva swimming about immediately before the ab sorption o f the s u s Pluteus . The series of figure found in this paper will give a more complete idea o f f s o f o f s s the di ferent pha es growth one specie of Echinu , than can be gathered from a s comparison even of the different species which Miille r h a investigated . It will enable u s o f s s s to trace the order appearance of the arms of the Pluteu , and the la t change which the larva undergoes immediately before the young Sea- urchin has resorbed the whole framework o f the Pluteus . The - process o f segmentation o f the yolk is entirely similar to what we observe in the Starfi sh ; the main differences in the eggs are simply o f proportion between the - s relative size of the yolk m ass and the outer envelope . My ob ervations agree with E ’ " o f the account of the segmentation given by D e rb é s. The formation the Rich tun g s - Blasch en is very easily followed in the Sea urchin . s s The yolk contract omewhat immediately after the fecundation takes place , and we o f might repeat here , word for word , the description the changes which the yolk und Starfish e s s o f s goes in the , and have the hi tory the change during the segmentation o f o f the yolk o f an Echinus . Fo r an account this I would refer the reader to the fifth o f N U volume the Contributions to the atural History of the nited States , by Professor ss . o n s s Aga iz The embryo , e caping from the egg , resemble a Starfish embryo , and it o n e f would greatly puzzle any to perceive any di ference between them . The forma o f o f a so h a us - s tion the stomach , the p g , the intestine , and the water tubes take place s f in exactly the same manner as in the Starfish , the time only at which the e di ferent s ff s organ are di erentiated not being the same. We have thu very early in the history o f these two orders differences which to a practised eye tell at once to which o f them f i the young larva belongs. What is a particularly important di ference s the forma tion in Ophiurans and in Echinoids o f calcareous rods at an early period o f the larval condition . Th e em r o a ter i ts esca e om th e e b y f p fr gg . In the spherical embryo soon after its escape from the egg we perceive a thicken o f o f s is ing the walls at one the poles ; a depres ion then formed at this extremity , s an d which become more and more marked ; T the wall then turns in , a small cavity ° E . i . D ER B n 3 III. 8 0 1 47 . S Ann (1 Sc e . Nat Ser V p 8 ‘ fi s. 4 5 1 m A ste racan th io n . VI . 1 4 1 8 63 See g and of y paper on in Proc Am Acad , Vol , April , , which s s repre ent the corre ponding stages of the embryo of the Sta rfish . ON THE EM B R YO LOGY OF E CH I N O D ER M S . 3 n s d is formed , which goes on i crea ing in length until we have a hollow cylinder ( ) f 1 2 s . o . 6 extending half the length the larva , as in figs and , which corre pond to fig . 1 of my paper referred to above . In the profile , fig , we notice the same tendency v in the digestive ca ity (d) to incline towards the lower side , after the dorsal portion l i vm b e re le d . has increased more rapidly, g g the anal part of the larva a appearance s . 3 In a omewhat older stage , fig , the digestive cavity is still longer , and almost touches f s - the lower side . We notice a di ference between the Starfi h and the Sea urchin in o f t s o s . the time formation of the alimen ary canal , the tom ach , and the e ophagus In the Starfish the mouth is formed before the differentiation o f these organs takes place ; while in the Sea- urchin the m outh is not formed until the alimentary canal and the - a so h a u s as s . p g , as well the water tubes , are quite di tinctly defined (See fig What is also peculiar to Echini is the pre sence of large masses of yolk- cells along the sides o f o f the digestive cavity , indicative the great changes which take place at the points - s where these masses of yolk cells are most numerous . We have ob erved that the yolk cells are always present wherever any new organ is developed ; in these larva the appearance of the water- tubes is preceded by an accumulation of yolk- masses at the f o f s . o . extremity the dige tive cavity (see fig and the place the limestone rods , fig ’ 4 r s s o f - in , , is first seen filled by clu ter these yolk masses , the midst of which the rods s a f are depo sited . Rod extending into the rms are characteristic o Echinoids and ra o f o r Ophiu ns ; we find nothing the kind in Starfish Holothurian larva . 4 d In the next stage , fig , the original cylindrical digestive cavity has already a e f d o f 0 cided tendency to di ferentiation , the walls of the stomach ( ) and the oesophagus ( ) o f f o being very di ferent thickness from , the pouches which are to become the water ' w w o f tubes ( ) project far beyond the outline the digestive cavity . The limestone ’ ’ rods (7 r ) can faintly be distinguished from the mass o f yolk - cells which surround them .

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