ANATOMV AND DEVELOPMENT OF THE ECPIINODERMS II3 



fringe and on one side. It is connected witli a vesicle which hes close 

 against the oesophagus, and from which the water-vascular system is 

 developed. 



At this place the shell of the Echinus sub.sequently makes its 

 appearance as a circular disc, which gradually envelopes the stomach, 

 and developes tentacles and spines. A new anus is formed as well as 

 a new oesophagus, in the young .sea-urchin. 



The development of the Ophiuridje has not been traced so far back 

 as that of the other groups. The dorsal 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 Asterida. 



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 Miiller's words, the \-ariations of the metamor- 

 phosis of Echinoderms : — 



" I. The change of the bilateral larva into the Echinoderm takes 

 place when the larva )'et remains an embryo, and is universally 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 Asterida;, Ecliinaster, Asteracanthion, Sars.) 



" 2. The change of the bilateral larva into the Echinoderm takes 

 place when the larva is perfectly organized ; that is, possesses digestive 

 organs and a special ciliated fringe. The Echinoderm is con.structed 

 within the Pluteus like a picture upon its canvas, 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 ^ {OpJimra, Echinus), 

 or is thro\vn off {Bipinnarid). 



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

 bilateral type with lateral ciliated fringe into the radial type, and re- 

 ceives instead of the previous ciliated fringe, new locomotive larval 

 organs, the ciliated ring,s. Out of this pupa-condition the Echino- 

 derm is developed without any part being cast off {Holothuria, some 

 Asteridse). 



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

 leaves the &gg, and when the internal organs are not j'et developed, we 

 have four stages or types— the embryonic type, the larval type, 'the 

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



1 It seems questionable how far the integument of the larva over the Echinoderm can be 

 said to vanish, when it is remembered that the pedicellarite are developed thereon while the 

 Echinoderm is still quite rudimentary. 



VOL. I J 



