438 Prof. Miiller on the Larval State and the Metamorphosis 



thinner. The uniform colour of the transparent animalcule was 

 a very pale violet. Its size was double that of Pluteus paradoxus. 

 There were no indications of its development into the star-fish. 



I now come to another class of Echinodermatous larvae, which 

 I have traced as far as the period of their metamorphosis, so that 

 there is no doubt in my mind regarding their Echinoid nature. 

 I had no opportunity of observing the earliest development of 

 the Echinus from the ovum, upon which point H. von Baer has 

 instituted investigations by the artificial impregnation of the 

 ova*. Von Baer compares the embryos of the Echinus to the 

 earliest forms of the larvse of the Medusa, such as Aurelia aurita, 

 as they occur in the sacs on the margins of the arms, except that 

 they are much broader. During their further change, they ap- 

 peared as if about to approach the Beroe in structure ; on the 

 fourth day they assumed very irregular forms which differed from 

 each other; on the fifth day they were all dead. The motion of 

 the larva when it has quitted the ovum is effected by cilia. Von 

 Baer estimates the diameter of the young Echinus which he ob- 

 served at jQ^ of a line. 



The animals which formed the subject of my investigations, and 

 which I consider as the larvse of Echini, are much older, about 

 half a line in diameter ; in this condition they have no resem- 

 blance to the larvse of the Meduscs and Beroe. I have observed 

 three kinds of the Echinoid larvse, two of which appear to be- 

 long to one and the same genus, the third to another genus of 

 Echini. 



One form, which I shall describe first, has an arched body, and 

 may be compared to a spheroid or dome with four columnar, some- 

 what divergent, elongated supports or feet. These calcareous co- 

 lumns are continued into the spheroid, where they are further 

 distributed in a peculiar manner, and which can only be rendered 

 intelligible by figures. The columns are covered by the skin of 

 the larva which forms the spheroid, and which forms arches at the 

 margin of the arch between the columns. The spheroid has two 

 broad and two narrow sides. The broad maybe distinguished as the 

 anterior and the posterior sides. Between the two anterior columns 

 the skin of the larva forms a tent-like expansion at the margin of 

 the spheroid ; on the opposite posterior side the animal substance 

 is continued from the dome into a long appendage, which is fixed 

 by four separate columns, so that there are two on each side. 

 This elongation contains the mouth and the oesophagus, the 

 stomach is situated beneath the dome. 



To assist comprehension by a comparison, the larva resembles 

 a clock-case resting upon four long feet, from the back of which 



* Bull, de I'Acad. Imp. de St. Petersb. t. v. n. 15. p. 2;31. 



