264 



NATURAL HISTORY. 



a large Starfish dredged by the Challenger in the Southern Seas, a sort of tent is formed in the middle 



of the upper surface of the body, which consists of five membranous valves supported by spines. 



These valves can be raised or drawn together so as to form a low pyramid ; and the eggs pass directly 



from the ovaries into its cavity, where they assume the form of young 

 Starfishes, without previously passing through the Bipinnaria and 

 Brachiolaria stages. In another species the spines covering the back 

 have flattened heads, which fit closely together, so as to cover in 

 the arcade-like spaces left between their shafts. The young develop 

 within these spaces, eventually pushing their way out by forcing the 

 spines aside. A similar nursery is formed on the back of a South Sea 

 Holothurian (Psolus) by the apposition of the heads of mushroom-shaped 

 plates ; while in another species from the Falkland Islands there is no 

 special nursery, but the young come to be packed into two continuous 

 fringes adhering to the two rows of tube-feet along the back, which are 

 imperfectly developed, and are not used for locomotion. In one South 

 American Holothurian, however, the young are protected within the 

 body of the mother, one individual having yielded sixteen young ones 

 measuring -J^ in length. In the viviparous Ophiurids, the nursery, 

 though internal, is not a portion of the body-cavity, but a pouch which 

 opens externally and projects into the body-cavity, serving also at the 



Fig. 6. DIAGRAM OF A SIDE VIEW same time as a breathing apparatus. There are usually ten of these 

 OF THE HOLOTHURIAN LARVA pouches, though as many as fourteen have been found in one individual 



REPRESENTED IN FIG. 5. . . ' 



n,, ni outh: , guiiet; , stomach; a, anus; each containing three young Brittle-stars. 

 of ^cvjScufarrii; > ^, ; witer^!re' Echinoderms are to be found in all parts of the ocean, whether 



rp, Ip, right and left peritoneal cavities, i i 



f mm which the water-vascuiar system is in polar, tropical, or temperate regions, but they are most varied and 



developed. (After Selenka.} 111 . 



on the whole, most abundant in the shallower waters of the tropical 

 seas. They have also a wide bathymetrical range, extending from between tide- 

 marks to some of the greatest depths explored by the dredge, where they are 

 chiefly represented by the Ophiurids. There are certain forms in each class 

 which are especially characteristic of the abyssal depths, and have a very 

 extensive distribution. Thus most deep-sea Holothurians belong to a very 

 remarkable section of the group, the Elasmopoda* which look singularly 

 like nudibranchiate Mollusca. The Stalked Crinoids are also characteristic of 

 the greater depths, some of them being the last survivors of a large and 

 important group (ApiocrinMfy which flourished in the Mesozoic Seas. Similarly, 

 the more prominent abyssal forms among the Sea-urchins are chiefly those 

 which have a flexible test (Asthenosoma), instead of a shell of immovable 

 plates. They belong to a very singular group, which was believed to have 

 become extinct after the deposition of the white chalk. Among the Starfishes 

 and Ophiurids, again, the same generic types inhabit the great ocean depths in 

 all parts of the world ; but they are not so interesting in their palseontological 

 relations as the Stalked Crinoids and the flexible Urchins. 



Fossil Echinoderms occur in most of the stratified rocks from the Upper 

 Cambrian upwards. Certain Palaeozoic limestone beds are almost exclusively 

 composed of crinoidal remains. The Stalked Crinoids were most abundant 

 during the Palaeozoic period, during which the Cystids* and Blastoids also 

 flourished, to become extinct at or before its close. But the free Crinoids 

 (Comatula\\) are probably more abundant at the present time than in any previous geological period. 

 Starfishes are among the earliest known Echinoderms, and appear to have gone on increasing in 

 importance from the Cambro-Silurian period until the present day. Little is known of the fossi] 



* Greek, clauno, to move ; pous, fool. T Greek, apion, a pear ; krinon, a lily. 



J Greek, kustis, a bladder; eidos, form. Greek, bhtstos, a bud ; eidos, form. 



|| Latin, coma, hair ; and the obsolete form, tulo, I bear. 



7. DORSAL VIEW OF 

 THE LARVA OF THE ROSY 

 FEATHEK-STAU (COMA- 

 TULA ROSACEA) SHORTLY 

 BEFORE THE DISAPPEAR- 

 ANCE OF THE CILIATED 

 BANDS. MAGNIFIED 



TWENTY TIMES. (After 



Wyville- Thomson. ) 



