EMBEYOLOGY OF OPHIUEA. 187 



(491.) The first appearance that presents itself, indicating the com- 

 mencement of metamorphosis, is the development of a number of 

 csecal appendages around the stomach and oesophagus of the Pluteus 

 (fig. 92, 1, d), which soon increase so much in number that they form 

 a series of rows surrounding the stomachal cavity. At first these rows 

 of cseca do not extend beyond the body of the Pluteus, remaining, as it 

 were, concealed beneath its disk ; but soon acquiring greater develop- 

 ment, they make their appearance externally, and begin to assume 

 some regularity of arrangement (fig. 92, 2), in which the rudimentary 

 form of the star-fish begins to be perceptible, and the points whence 

 the arms are to proceed become apparent. 



(492.) In carrying out this part of the proceeding, it will be ob- 

 served that the original arms or processes of the Pluteus (fig. 92, 1, A, B, 

 c, D) have had no share. The Pluteus, in fact, stands just in the same 

 relation to the young Ophiura as the frame does to a piece of embroi- 

 dery ; neither has the structure of its arms anything in common with 

 that of the rays of the future star-fish, which lies, as it were, protected 

 beneath their shelter. As soon as the csecal appendages have arrived 

 at this state of development and assumed so much regularity of 

 arrangement, calcareous earth begins to be deposited in an arborescent 

 form, which accumulates rapidly until a kind of trellis-work is formed, 

 spreading over the entire surface of the young Echinoderm. As the 

 caDciform appendages thus become arranged into a regular figure, the 

 place where the mouth of the Pluteus was becomes distorted and, as 

 it were, forcibly pushed upwards, until it remains no longer visible, its 

 place being occupied by the central mouth of the newly-formed star- 

 fish (fig. 93, 2). 



(493.) In the condition which it has now attained, the young star- 

 fish is still much smaller than the rest of the Pluteus ; but from 

 this point, as its growth continues, the body and processes of the latter 

 assume more and more the appearance of being only appendages to 

 the newly- developed animal, until by degrees they entirely disappear, 

 the only part of the Pluteus remaining as a part of the young Ophiura 

 being the stomach. 



(494.) Before, however, the arms of the Pluteus have entirely dis- 

 appeared, the feet, or retractile suckers, have begun to show themselves, 

 arranged in a circle around the circumference of the shield (fig. 93, 1, 2), 

 so that it is able to creep freely about in the sea. 



(495.) Shortly before the disappearance of the last remnants of the 

 Pluteus, the arms or rays of the Ophiura are already visible, projecting 

 prominently from the margin of the shield (fig. 93, 1, 2), but consisting 

 as yet only of the outer or terminal joint of the future ray; the moveable 

 spines likewise begin to show themselves, and the characters of the 

 future Echinoderm begin to be recognizable (fig. 93, l). Ultimately 

 new segments begin to be added to the rays, making their appearance 



