89. Magnified head of Fig. 88 (Agassiz) 68
97. Young Medusa just freed from the Hydroid 71
99. Cluster of Medusæ of Fig. 98 (Agassiz) 72
100. Female colony of HYDRACTINIA POLYCLINA (Agassiz) 73
101. Male colony of the same (Agassiz) 73
102. Unsymmetrical Medusa of HYBOCODON PROLIFER (Agassiz) 74
103. Medusa bud of Hybocodon (Agassiz) 74
106. Proboscis of Fig. 105 with young Medusæ 75
109. Nanomia somewhat older than Fig. 108 77
110. Heart-shaped swimming bell of Nanomia 77
111. Cluster of Medusæ with tentacles having pendent knobs 78
113. Medusa with corkscrew-shaped tentacles 79
129. Cuvieria somewhat older than Fig. 128 99
132. Portion of shell of Fig. 131 without spines (Agassiz) 103
133. Sea-urchin shell without spines (Agassiz) 103
134. Sea-urchin from the mouth side (Agassiz) 104
140. Transverse section of Echinarachnius (Agassiz) 108
141. Ray of Star-fish, seen from mouth side (Agassiz) 109
144. Limestone network of back of Star-fish 111
149. Arm of Fig. 148, from the mouth side (Agassiz) 116
155, 156, 157. Egg of Star-fish in different stages of development 124
159-164. Successive stages of development of Larva 125
165. Larva in which arms are developing 126
166. Adult Star-fish Larva (BRACHIOLARIA) 127
168-170. Young Star-fish (Astracanthion) in different stages of development 129
171. Lower side of ray of young Star-fish 130
172. Very young Star-fish seen in profile 130
173-175. Larvæ of Sea-urchin (Toxopneustes) in different stages of development 130, 131
178. Sea-urchin resorbing the arms of the larva 133
179-181. Successive stages of young Sea-urchin 133, 134
182. Ophiuran which has nearly resorbed the larva 135
185. Cluster of eggs of Star-fishes over mouth of parent 137
It is perhaps not strange that the Radiates, a type of animals whose home is in the sea, many of whom are so diminutive in size, and so light and evanescent in substance, that they are hardly to be distinguished from the element in which they live, should have been among the last to attract the attention of naturalists. Neither is it surprising to those who know something of the history of these animals, that when the investigation of their structure was once begun, when some insight was gained into their complex life, their association in fixed or floating communities, their wonderful processes of development uniting the most dissimilar individuals in one and the same cycle of growth, their study should have become one of the most fascinating pursuits of modern science, and have engaged the attention of some of the most original investigators during the last half century. It is true that from the earliest days of Natural History, the more conspicuous and easily accessible of these animals attracted notice and found their way into the scientific works of the time. Even Aristotle describes some of them under the names of Acalephæ and Knidæ, and later observers have added something, here and there, to our knowledge on the subject; but it is only within the last fifty years that their complicated history has been unravelled, and the facts concerning them presented in their true connection.