ECHINODERMATA. 221 



structed within the Pluteus, like a picture upon its canvas, or 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 dis- 

 appears {Ophiuray Echinus), or is thrown off (Sipinn aria), 



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

 bilateral type with the 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 olF. (Holothuria and some 

 Sielleridce)."* 



Prof. Mliller, whilst prosecuting his researches into the develop- 

 ment of the soft vermiform species of Echinodermata, at Trieste, in 

 August, 1851, was further rewarded by the discovery of a very 

 singular phenomenon. In upwards of seventy instances he found in 

 the Synapta digitata a tube, one half green, the other half yellow, 

 and in more than twenty instances this tube was organically con- 

 nected by both ends with the Synapta, — viz. to the head by its 

 yellow half, where the tube opens externally, and to the intestine by 

 its green half, which contains an intus-suscepted portion like the in- 

 verted finger of a glove. The vessel which runs along the free side 

 of the intestine gives off a branch which surrounds the open end of 

 the intus-susception. Where the sac is embraced by the vessel it is 

 enlarged into a sort of knob, from the middle of which the involuted 

 portion passes into the interior of the sac. The place of attachment is 

 a short way behind the muscular stomach. Sometimes there are two 

 or three such tubes so attached one behind the other : their outer sur- 

 face is not ciliated, like the ordinary branched ovarian tubes of the 

 Synapta : they exhibit slow vermicular motions executed by a 

 muscular coat consisting of transverse and longitudinal fibres ; the 

 inner surface, save at the green portion^ presents a lively ciliary 

 motion, and in this ciliated part the sperm-sacs and ovarium lie free. 

 The ovarium is a closed tubular capsule, ciliated upon its whole outer 

 surface : it contains orange-coloured ova, attached to short branches 

 from a central stem : each ovum consists of a yolk, and a germinal 

 vesicle ^^th of a line in diameter : when fully developed the ovarian 

 tubule is rent, and the ova escape into the common tube. The sperm 

 sacs are from eight to eighteen in number, and lie free in a rather 

 wide portion of the common tube : they are lined by a layer of cells, 

 and are filled with Spermatozoa. These are liberated by solution of 

 their formative sacs, and impregnate the similarly free ova. The 

 progressive geometric fission of the yolk follows : a ciliated embryo 



* CLXXIV. p. 33. 



