508 LARVAE OF ECHINODERMATA. 



Potass ; and they should be mounted in Canada Balsam. But 

 their position in the Skin can only be ascertained by making 

 sections of the Integument, both vertical and parallel to its surface ; 

 and these Sections, when dry, are most advantageously mounted 

 in the same medium, by which their transparence is greatly in- 

 creased. All the objects of this class are most beautifully displayed 

 by the Black-Ground illumination (§§ 84-86) ; and their solid 

 forms are seen with increased effect under the Binocular. The 

 Black-Ground illumination applied to very thin sections of Echinus 

 Spines brings out some effects of marvellous beauty ; and even 

 in these the solid form of the network connecting the pillars 

 is better seen with the Binocular than it can be with the ordinary 

 Microscope. * 



440. Echinoderm-LarvcB. — We have now to notice that most 

 remarkable set of objects furnished to the Microscopic inquirer by 

 the Larval states of this class ; for our present knowledge of which, 

 imperfect as it still is, we are almost entirely indebted to the 

 painstaking and widely-extended investigations of Prof. Miiller. 

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

 forms of these Larva?, by way of sample of the wonderful pheno- 

 mena which his researches have brought to light ; so as (it may be 

 hoped) to excite such an interest among those Microscopists in 

 particular who may have the opportunity of pursuing these in- 

 quiries, as may induce them to apply themselves perseveringly to 

 them, and thus to supply the numerous links which are at present 

 wanting in the chain of Developmental history. — 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 

 Zooid or Pseudembryo, which seems to exist for no other purpose 

 than to give origin to the Echinoderm by a kind of internal gem- 

 mation, 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 accumulation of their pro- 

 geny. 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 ciliated 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 



* It may be here pointed out that the Reticulated appearance is some- 

 times deceptive ; what seems to be a solid network being in many in- 

 stances a hollow network of passages channelled-out in solid Calcareous 

 substance. Between these two conditions, in which the relation between 

 the solid framework and the intervening space is completely reversed, 

 there is every intermediate gradation. 



