150 THE MICROSCOPE AND ITS REVELATIONS. 



greatly increased. All the objects of this class are most beautifully dis- 

 played by the Black-ground illumination; and their solid forms are seen 

 with increased effect under the Binocular. The Black-ground illumina- 

 tion applied to very thin sections of Echinus spines brings out some 

 effects of marvellous beauty; and even in these the solid form of the net- 

 work connecting the pillars is better seen with the Binocular than it can 

 be with the ordinary Microscope. 1 



542. EcUinoderm-LarvcB. We have now to notice that most remark- 

 able set of objects furnished to the Microscopic inquirer by the larval 

 states of this class; for our knowledge of which we are chiefly indebted 

 to the painstaking and widely-extended investigations of Prof. J. Miiller. 

 All that our limits permit is a notice of two of the most curious forms of 



| these larvae, by way of sample of the wonderful phenomena which his 

 i researches brought to light, and to which the attention of Microscopists 

 I who have the opportunity of studying them should be the more assid- 

 uously directed; as even the most delicate of these organisms have been 

 found capable of such perfect preservation, as to admit of being studied, 

 when mounted as preparations, even better than when alive ( 545, a). 

 The peculiar feature by which the early history of the Echinoderms 

 generally seems to be distinguished is this, that the embryonic mass of 

 cells is converted, not into a larva which subsequently attains the adult 

 form by a process of metamorphosis, but into a peculiar e zooid ' or 

 pseudembryo, which seems to exist for no other purpose than to give 

 origin to the Echinoderm by a kind of internal gemmation, and to carry 

 it to a distance by its active locomotive powers, so as to prevent the spots 

 inhabited by the respective species from being overcrowded by the accu- 

 mulation of their progeny. 1 The larval zooids are formed upon a type 

 quite different from that which characterizes the adults; for instead of a 

 radial symmetry, they exhibit a bilateral, the two sides being precisely 

 alike, and each having a ciiliated fringe along the greater part or the 

 whole of its length. The two fringes are united by a superior and an 

 inferior transverse ciliated band : and between these two the mouth of 

 the zooid is always situated. Further, although the adult Star-fish and 

 Sand-stars have usually neither intestinal tube nor anal orifice, their 

 larval zooids, like those of other Echinoderms, always possess both. 

 The external forms of these larvae, however, vary in a most remarkable 

 degree, owing to the unequal evolution of their different parts ; and 

 there is also a considerable diversity in the several Orders, as to the pro- 

 portion of the fabric of the larva which enters into the composition of 

 the adult form. In the fully developed Star-fish and Sea-urchin, the 

 only part retained is a portion of the stomach and intestine, which is 

 pinched-off, so to speak, from that of the larval zooid. 



543. One of the most remarkable forms of Echinoderm-larvae is that 

 which has received the name of Bipinnaria (Fig. 376), from the symme- 

 trical arrangement of its natatory organs. The mouth (), which opens 

 in the middle of a transverse furrow, leads through an oesophagus a' to a 

 large stomach, around which the body of a Star-fish is developing itself; 

 and on one side of this mouth are observed the intestinal tube and anus 

 (b). On either side of the anterior portion of the body are six or more 



1 It may be here pointed out that the reticulated appearance is sometimes de- 

 ceptive; what seems to be solid network being in many instances a hollow net- 

 work of passages channelled-out in a solid calcareous substance. Between these 

 two conditions, in which the relation be ween the solid frame- work and the inter- 

 vening space is completely reversed, there is every intermediate gradation. 



