20 



In the course of four days this animalcule had become me- 

 tamorphosed into (or had given birth to ?) a minute trans- 

 parent acalephbid creature which he compares to a Beroe. 

 Here his observations ceased, the animals having perished 

 on the fifth day. 



In the autumn of 1845 Prof. Miiller had discovered in 

 the sea at Heligoland a transparent acalephoid animalcule,, 

 referable by its mode of motion to the e ciliograde ' order, 

 but which, from its singular form, somewhat resembling 

 a painter's easel, he described under the name of Pluteus 

 paradoxus*. 



Returning in the following year to the same field of ob- 

 servation, he pursued with admirable detail the change of 

 this species into an Ophiura (Brittle star-fish), and that of 

 another kind of Pluteus into an Echinus^. 



The larva of the Echinus, like that of the Ophiura, has a 

 quadrilateral pyramidal body, of a colourless, transparent, 

 or hyaline substance, dome-shaped above, slightly excavated 

 at the base, the corners of which are prolonged into straight 

 slender legs, strengthened by filiform rods of calcareous 

 matter, reaching to the summit of the dome : the mouth is 

 prolonged from the middle of the concave base into a four- 

 sided proboscis, the angles of which are also produced into 

 slender processes shorter than the four outer legs. Were 

 the creature to stand upon these it would resemble a table- 

 clock case, and the proboscidiform mouth and appendages 

 might swing to and fro like the pendulum. The larva is, 

 however, a free-swimming animalcule, and propels itself, 

 with the base and processes forwards, chiefly by means of 

 strong cilia, grouped on two tubercles at the sides of the 

 dome, which are compared to epaulets. In the centre of the 

 dome-shaped body is a subspherical granular mass, which 



* Archiv fur Anat. und Physiol. 1846, p. 101. 



f ' Ueber die Larven und die Metamorphose der Ophiuren und See- 

 igel/ Vorgetragen in der Kbn. Akad. der Wissenschaften, 29 October 

 1846; 4to, 1848. 



