q/" Me Echinoderms. 11 



appearance as a circular disc, which gradually envelopes the 

 stomach, aud developes tentacles and spines. A new anus is 

 formed as well as a new oesophagus, in the young sea-urchin. 



The development of the Ophiuridse has not been traced so far 

 back as that of the other groups. The doi*sal pore and tube have 

 not been observed ; but the development of the " rosette " and 

 its accompanying mass of cells into the Echinoderm takes place 

 as in the Astcridse. 



The observations of Dr. Busch (Miill. Arch. 1849) have shown 

 that the larva of Comatula very early assumes the form of the 

 Holothurid-larva with ciliated rings, but its internal structure 

 and the development of the Echinoderm are not understood. 



To sum up, in Prof. ^liiller's words, the vai'iations of the me- 

 tamorphosis of Echiuoderms : — 



"1. The change of the bilateral larva into the Echinodenn 

 takes place when the larva yet remains an embryo, and is uni- 

 versally covered with cilia, without a ciliated fringe. A part of 

 the body of the larva takes on the form of the Echinoderm ; the 

 rest is absorbed by the latter. (A part of the Asteridse, Echi- 

 naster, Asteracanthion, Sars.) 



'' 2. The change of the bilateral larva into the Echinoderm 

 takes place when the lana is perfectly organized ; that is, pos- 

 sesses digestive organs and a special ciliated fringe. The Echi- 

 noderm is constructed within the Pluteus like a picture upon its 

 canvass, a piece of embroidery in its frame, and then takes up 

 into itself the digestive organs of the larva. Hereupon the rest 

 of the larva vanishes* {Ophiura, Echinus), or is thrown off {Bi- 

 pinnaria). 



" 3. The larva changes twice. The first time it passes out of 

 the bilateral type with lateral ciliated fringe into the radial type, 

 and receives instead of the previous ciliated fringe, new locomotive 

 larval organs, the ciliated rings. Out of this pupa-condition the 

 Echinoderm is developed without any part being cast off {Holo- 

 tkuria, some Asteridse). 



" If we call embryonic type the condition in which the animal 

 leaves the egg, and when the internal organs are not yet deve- 

 loped, we have four stages or types — the embryonic type, the larval 

 type, the pupa type, and the Echinoderm type. The animal may 

 pass from either of the first three forms into the Echinoderm, 

 or may run through them all." (Larvcn u. Mctam. d. Holoth. u. 

 Asterien, p. 33.) 



Furthermore it may be stated that the nature of the change 



* It seems questionable how far the integument of the larva over the 

 Echinoderm can be said to vanish, when it is remembered that the jiedi- 

 cellaria; arc developed thereon while the Echinoderm is still quite rudi- 

 mentan". 



