68 



INTRODUCTION TO ZOOLOGY 



more important in past ages ; now it consists of a few forms 

 only, living at great depths in the sea. They are nearly 



all sedentary, consisting of calcare- 

 ous disc-like bodies, with branched 

 segmented arms surrounding the 

 mouth on the upper side of the body. 

 In these cases the discs are attached 

 to some object by a segmented cal- 

 careous stalk, as in Pentacrinus, one 

 of the stalked "Sea-lilies." Some 

 of them are stalked only when 

 young, and move freely about when 

 adults with a creeping or gentle 

 swimming motion ; such forms are 

 found in comparatively shallow 

 water — the Eosy Feather Star 

 (Antedon rosacea) may be dredged 

 up from a depth of only ten fathoms, 

 round the south-west coast of 

 England. Fig. 36 shows this form 

 in its young stage when it is still 

 attached by a stalk. 

 Echinoderm ^^^ Echinoderms have 

 develop- a very peculiar develop- 

 ment, ment. From the egg 

 arises a minute, ciliated, free-swimming larva, with bilateral 

 symmetry, which only gradually, by a very peculiar form of 

 metamorphosis, becomes converted into the adult form with 

 radial symmetry. The study of the development is specially 

 interesting, for it gives indications of the directions in which 

 we may look for links between Echinoderms and other animals, 

 but it is a study beyond the scope of this book, and the student 

 is therefore referred to more comprehensive works.^ 



In many ways the Echinoderms form a very peculiar 

 group, the relationships of which it is diflScult to trace 

 with any certainty. An interesting probable link, however, 

 between them and the Vertebrates may be seen in the 

 curious burrowing worm-like form Balanoglossus, which has 

 a ciliated free-swimming larva almost identical with that 



Fig. 36. — Antedon rosacea. 

 Young stalked stage. (From 

 the British Museum Guide 

 to the Starfish Gallery. ) 



' The Oambridge Natii/ral History, vol. 1. chap. xxi. 

 Zoology, hy T. J. Parker and W. A. Haswell. 



or A Text-Book of 



