176 ECHINODERMAIA. PART in. 



alternately a fixed and a free state. Mr. J. V. Thomson 

 discovered that the Pentacrinus Europseus is merely the 

 fixed state of a Comatula. These star-fishes have pairs 

 of pinnae placed at regular distances along their long- 

 jointed rays, and in the pinnae sacs containing eggs are 

 placed as far as the fifteenth or twentieth pair. The 

 eggs yield active ciliated larvae, which attach themselves 

 in the form of flat oval disks to corallines and sea- weeds. 

 By degrees they develop a stem, about three-fourths of 

 an inch high, with twenty-four distinct joints. Its ex- 

 panded top bears five sulphur-coloured bifurcating rays 

 with their pinnae and dorsal cirri. A mouth is formed 

 in the centre with its tentacles, and a lateral prominent 

 vent. The actual change of a Pentacrine into a Oo- 

 matula has not been seen, but as the small Pentacrinites 

 disappear in September, at which season the Comatulse 

 appear, it is believed that when full grown the top of 

 the fixed Pentacrinite falls off and becomes a Comatula, 

 which swims backwards with great activity by striking 

 the water alternately with its long rays. The Penta- 

 crinus caput-Medusae, which is fixed by its stem to sea- 

 weeds and zoophytes, forms a most beautiful object for 

 the lower magnifying powers when viewed in a fluid by 

 a strong refracting light. 



Echinodermata Echino'idea. 



The family of Echinidae, commonly known as Sea-Eggs 

 or Sea-Urchins, have a beautiful but complicated struc- 

 ture. The calcareous shell of an Echinus is a hollow 

 spheroid with large circular openings at each pole. In 

 the larger of the two, called the corona, the mouth of the 

 animal is situated ; in the lesser circle the vent is placed. 

 The spheroid itself is formed of ten bands extending in a 

 meridional direction from the corona to the lower ring ; 

 that is, from one polar circle to the other. Each band 

 consists of a double row of pentagonal plates increasing 



