LECTURES ON EMBRYOLOGY 



13 



have been foundbetween the fossils and the 

 living ones, that we shall have an opportunity to 

 allude to another relation which exists between 

 .these different ^forms. Those which have existed 

 earliest upon our globe, in the ancient geological 

 epochs, do not indeed resemble those which live 

 now ; but they are related to the forms of the Ech- 

 inoderms of the present day in their earlier stages 

 of growth). And so the class of Echinoderms will 

 afford us the means of investigating all the differ- 

 ences which exist between the animals of that class 

 living now, as compared with their embryonic 

 changes, and also between the changes which the 

 representatives of the same class have undergone 

 from the earliest geological times, up to the time 

 when theorderof things which now prevails upon 

 this globe was introduced. 



But yet very little was known of the embryology 

 of Echinoderms. Two singular investigations had 

 been made upon this subject, one by Mr. Thomp- 

 son, of Cork, who had ascertained that the Coma- 

 tula, a star fish with pinnate rays, of which you 

 have here a figure [Plate I,fig. B] produces youngs 

 like this [Plate I, fig. A], resting upon a slen- 



PLATTC T TIGS A AND B. 



der stem, which during their growth cast this stem, 

 become free, and assume finally the appearance of 

 Fig. B. 



Next, a Norwegian naturalist, Mr. Sars, traced 

 the changes which the egg of the Star-fish under- 

 goes. Here are the different figures which Sars 

 drew of the young of a small species of Star-fish 

 called Echinaster Sarsii, which occurs on the Nor 

 wegian coast. It is first a spheroidal mass, which 

 [PLATE II SAKS YOUNG STAR-FISHES.] 



is said to move free, like Infusoria, when upon 

 one of its surfaces three tubercles are first observ- 

 ed. [Plate K, fig Aj. 



These tubercles soon become more extensive and 

 run together, forming a figure, similar to a Roman 

 T. [Fig.B]. 



Here it is in profile, [Fig. C] where the cross 

 of Fig. B. appears like two horns on the upper side. 

 This prominent part next assumes this figure [Fig. 

 D] and seen in profile, it is like the letter E. After 

 this the sphere is divided into five lobes, [Fig. F] 

 with a central one more prominent. Finally, that 

 figure would become more and more flat [Fig. G] 

 its prominent horns which had grown larger, are 

 afterwards reduced, and finally disappear entirely, 

 and an animal similar to a Star-fish is produced. 



From these investigations, Sars concluded that 

 the young star-fish was originally a spherical being* 

 swimming free like the infusoria that it soon as- 

 sumed a bilateral form, and that this was finally 

 changed to a star form. In this I think Sars has 

 been mistaken, in as far as the bilateral outlines of 

 the young as he represents it, is only the result of 

 a lateral flexion of the peduncle hanging under the 

 centre of the umbrella-shaped little animal. 



But in order to show how a simple egg is trans- 

 formed into an animal so complicated as the star- 

 fish, it is now necessary for me to allude, first, to 

 the structure of Echinoderms in general. It would 

 be otherwise impossible for me to show how the 

 various parts are gradually developed, if I could 

 not refer to the complicated organization of the full 

 grown animal. These details would* indeed have 

 very little Interest if they were not described in 

 connexion with the complicated structure of the 

 perfect animal. 



[PLATE III GERMS OF STAR-FISHES.] 



[PLATE IV YOUNG STAR-FISHES.] 



VERTICAL SECTIONS. 



