ECHINODERMS. 135 



are situated pear-shaped vesicles, collected in some species into 

 bunches, which TIEDEMANN conceives to be male genital organs, 

 an opinion which falls to the ground now that the sexes are known 

 to be distinct. Moreover they do not lead into this canal, but are 

 in connexion with the oesophagus. It must be noted as a remark- 

 able exception that in Synapta, according to the investigations of 

 QUATREFAGES, a complete hermaphroditism prevails. The genital 

 organs have the form of long strings, whose internal surface is beset 

 with conical structures containing Spermatozoa, whilst the inner- 

 most cavity is filled with a pulpy substance in which the eggs are 

 found. These eggs, as they grow, are pressed against the conical 

 structures and so fertilized ; and then the germ-spot, which was 

 before visible, disappears. As the development of the eggs pro- 

 ceeds, the testes which adhered to the inner wall of the string and 

 surrounded the eggs, are so much compressed that they become 

 atrophied and disappear. This periodical development is a very 

 remarkable physiological phenomenon 1 . 



With the exception of some interesting observations of SARS 

 little was known respecting the development of Echinoderms until 

 the last few years. [To the distinguished and unremitted labours of 

 MUELLER we are principally indebted for nearly complete informa- 

 tion respecting the very curious and unexpected processes which 

 occur in different species. Very remarkable differences are observed 

 according as more or less of the development is effected within the 

 body of the parent, or according to the locality where the embryo 

 is deposited on leaving the egg, or according to the different modes 

 in which it is destined to acquire its food. As a general rule it 

 may be stated that in littoral species when the embryo escapes at 

 an early period from the egg the series of metamorphoses is less 

 numerous: but that in pelagic species, where the embryo has to 

 seek its food by swimming on the surface, the necessity for provi- 

 sional organs of a complicated nature renders the changes very 



1 Ann. des sc. nat., sec. seV. xvii. 1842. Zool. pp. 66, 73. A talented observer, whom 

 we have already quoted when treating of Polyps, (p. 70.) STEENSTRUP has with much 

 acuteness endeavoured to reject ffermaphroditism altogether, and is of opinion, that 

 even here QUATREFAGES has taken cells of spermatozoa for eggs without the germinal 

 spot. Undersoegelser over Hermaphroditismus Tilvaerelse i Naturen, Kjobenhavn, 

 1845, 4to, pp. 63, 64. (SIEBOLD also surmises the same mistake of QUATREFAGES.) We 

 shall perhaps recur subsequently to STEENSTRUP'S opinion. 



