262 



NATURAL HISTORY. 



from the spherical condition into one which exhibits a very complete bilateral symmetry. Portions 

 of the primitive digestive tube become cut off from it, and form closed " vaso-peritoiieal " cavities 

 (Figs. 2, 4, 6, w), which develop into the body-cavity and the water-vascular system of the mature 



Echiiioderm. The latter system acquires a communication with the 

 exterior by a water-tube, which opens 011 the dorsal surface of the body 

 by a " water-pore " (Figs. 2, 6, wjj), that eventually becomes the madre- 

 porite of the Echinozoa. 



The forms assumed by the fully-grown Echiiioderm larvae are very 

 various. In the Urchins (Fig. 2) and Ophiurids (Fig. 3) the dorsal region 

 of the larva is produced into a sort of conical hump, while the ventral 

 face becomes much excavated, and its edges are produced into four pairs of 

 slender processes, or arms, which are symmetrically arranged around the 

 mouth (Fig. 2, m). These arms are supported by a framework of lime- 

 stone rods, which has somewhat the appearance of an inverted painter's 

 easel, and is exclusively characteristic of the Ui'chins and Ophiurids. It 

 has nothing to do with the skeleton of the mature Echinoderm, coming 

 into existence before this makes its appearance, and disappearing as it 

 attains its full development. The arms open and shut like the ribs of 

 an umbrella during the movements of the larva, which is generally 

 known as a " Pluteus." This name was given to it on account of its 

 fancied resemblance to a painter's easel. 



In the Crinoids, on the other hand, a somewhat complicated skeleton 

 makes its appearance at a very early stage of embryonic life, the whole 

 or greater part of which passes, directly into the skeleton of the adult. 

 Fig. 2. PLUTEUS LARVA OF THE But in the larvse of Starfishes and Holothurians there is little or no 

 *ivfdu^ sEEx^ """* provisional skeleton ; and the rudiments of the skeletal system of the 

 m. mouth; a, anus; top, water-pore, or adult do not appear till the later stages of larval existence. The two 



external opening of the water-tube. .|. . i i n n * n m < i_ i in i 



which leads into w, the rudiment of tnj ciliated bands at first encircling the Starfish larva gradually extend 



water, vascular ring. (Ajttr Mvtschuikoff.) , , 



themselves, until they enclose nearly the whole or the upper and lower 

 halves of the body, so as to form two large crescentic shields. 

 The larva in this condition exhibits complete bilateral sym- 

 metry, and is called " Bipinnaria."* Loops now appear in 

 the outlines of the oral and anal shields, and gradually increase 

 in length, so as to develop into a number of long slender 

 movable arms, which stretch out from the larval body in 

 various directions, bending and twisting in the most graceful 

 manner, as they are not supported by limestone rods (Fig. 4). 

 The continual play of these arms not only assists in the loco- 

 motion of the larva, but also produces currents in the water 

 which set towards its mouth. This stage of the Starfish larvfe 

 is known as the " Brachiolaria" ;f and the development of the 

 adult Echinoderm from it or from a Pluteus takes place 

 entirely at the hinder end of the larval body. Rudiments of 

 tentacles appear on the growing water-vascular ring, which is 

 situated at the left side of the larval stomach, while the first 

 traces of the permanent skeleton show themselves on its right 

 side, near the dorsal pore of the water-vascular system. The 

 remainder of the larval body gradually shrivels up and dis- 

 appears, its substance going to feed the growing Echinoderm. 

 This is well shown in Fig. 4, which represents a Brachiolaria 

 with the Starfish disc developed at its anal extremity, some 

 of its arms having been already absorbed. A similar process goes 011 in the case of the Pluteus 

 * Latin, bis, twice ; pinna, a feather. f Latin, brachiolam, diminutive of Irachium, an arm. 



3. Pluteus paradoxm, THE ADVANCED 



LARVA OF AN OPHIURID, "WHICH CONTAINS 

 THE RUDIMENTS OF THE DISC AND RADIAL 

 SKELETON OF THE ADULT, TOGETHER WITH 

 THE LIMESTONE FRAMEWORK THAT SUPPORTS 



THE ARMS OF THE PLUTEUS. (After J. MiUler.) 



