LECTURES ON EMBRYOLOGY. 



19 



[PLATE Xlf SAKS' YOUNO OPFTTTTRA.] 



The same observer afterwards found, that within 

 this curious frame there was forming a sack, with 

 an external opening hanging down between the 

 longer lateral sticks. (Plate XII, fig. A.) He 

 describes the opening, (Plate XII, fig. B), as a 

 mouth, the tube above as an oesophagus or ali- 

 mentary canal emptying into a sack, which be 

 calls a stomach. (Plate XII, fig. A.) The mouth 

 hangs here lowest, the tube above being an oeso- 

 phagus, and the sack in the centre a kind of sto- 

 mach. The changes which gradually take place 

 in this animal, and which are represented, Plate 

 XII, from A to F, were noticed, not by tracing one 

 and the same individual, but by comparing the dif- 

 ferences between those which were successively 

 fished up from time to time, as it was impossible 

 to trace for a long time the same animal, owing 

 to the fact of their dying away very rapidly. 

 Comparing various individuals, Muller ascertained 

 that on the sides of the inner sac or stomach, 

 there were little processes or csecal appendages 

 arising (Plate XII, fig. B) from the side, which 

 grew out of two sides of that cavity which he con- 

 sidered as a stomach ; and these appendages grow- 

 ing more numerous, would form finally a bunch in 

 the centre, (Plate XII, fig C) consisting of about a 

 dozen of such rounded masses distinctly developed 

 from the sides of the stomach. Next there would 

 be (Plate XII, fig. D) a regular arrangement of the 

 growing protuberances arising from five definite 

 points, two and two, projecting more than the oth- 



ers from each of these points, and from that time, 

 an indication of the starfish, forming within this 

 curious stage, is clearly noticed. 



A regular star-fish has five beginning rays, en- 

 closed between those stems, developed from that 

 hollow organ which in the beginning is the simple 

 sack in its interior, with a wide opening on one 

 end, which gradually disappears in the new ani- 

 mal. At this epoch the young animal has no 

 opening at all ; what was was first considered as a 

 mouth is shut up (Plate XII. fig- C). After a cer- 

 tain time, however, upon one of the surfaces will 

 be found a new opening, (Plate XII fig. E). The 

 rays are advancing, growing longer by the ad- 

 dition of some new divisions in the mass ; and 

 growing larger and longer, the rays would become 

 soon very prominent, and suckers like those of the 

 little star fishes which I have described, would 

 come out, when a real mouth is seen in the centre, 

 and no indication as to what has become of the 

 curious tube first considered as an oesophagus, 

 (Fig. B). The surrounding transparent frame has 

 been reduced to a few processes, to a few append- 

 ages on the dorsal surface of the animal, (Fig. E). 

 They are afterwards still further reduced, only a 

 few remaining appended to the dorsal surface ; and 

 at last we have an animal entirely deprived of such 

 appendage, (Plate XII. fig. F). Out of such an 

 envelope will finally grow an ophiura, (Plate XVI.) 



[PLATE XVI OPHIURA. j 



an animal in which there is no indication of dorsal 

 appendages, a regular star-fish, with slender cylin- 

 drical arms. It is easy to see how the central mass 

 is transformed into the star-fish, from the period 

 (Plate XII. fig. C) when the inner pouch has been 

 transformed into a spherical mass of globules. 

 But the whole series of changes can scarcely be 

 reconciled to what I have observed in the common 

 star-fish of these shores. Perhaps there is some 

 resemblance between the sac of yolk of the star- 

 fish with its peduncle below (PI III fig.A.p.13,} and 

 the so-called stomach oesophagus and mouth of 

 the ophiura, (Plate XII. A B). Perhaps instead 

 of being a stomach with an oesophagus and mouth, 

 this inner mass is to be considered as a yolk with 

 a peduncle. But whether it be so or not, the dif- 

 ference is nevertheless striking. The young 

 ophiura, when forming, is here (Plate XII ) sur- 

 rounded by a peculiar frame, of which there ia not 

 any indication in my star-fish, (PI IV. fig. C,p. 13). 



