30 ASCIDIAD.^, 



In 1828 Milne-Edwards and Audouin* made the im- 

 portant discovery that the Compound Ascidians did not be- 

 gin their hfe as fixed animals, but originated from indepen- 

 dent tadpole-like embryos. In 1835 a similar account of 

 their early history was given by the Norwegian naturalist, 

 Sars ;-f- and in 1839 Sir John Graham Dalyell,| of Edin- 

 burgh, published his observations on the development of 

 Ascidice, both simple and compound, with the same results. 

 Since then many observers have noticed the metamorphoses 

 of the Tunicata — metamorphoses which account for the 

 wide diffusion of these apparently sedentary animals. The 

 tadpole as it appears in the egg is at first an oval disk ; a 

 tail is soon after observed; arm-Hke projections spring from 

 the head of the creature, which then presents a striking 

 analogy with the form of a hydroid zoophyte ; it becomes 

 free and swims about by means of its rapidly vibrating 

 tail ; it fixes itself to rocks or sea-weeds by its arms ; the 

 tail disappears ; that which was the head, or nucleus, 

 sends out root-like projections ; orifices appear in it, and 

 its final form as an Ascidian begins to be manifested. 

 Such are the successive stages of the metamorphosis. § 



ASCIDIA, Baster. 

 (a(rxo;, a leather bag.) 



Body sessile, covered with a coriaceous or gelatinous tunic. 

 Branchial orifice 8-lobed and 6-lobed. [Branchial sac not pli- 

 cated, surmounted by a circle of simple tentacular filaments; 

 meshes of the respiratory sac papillated,] This is the genus 

 Phallusia of Savigny. 



* Annales des Sciences Naturales, t. xv. p. 10. 

 f Sars, Beskrivelser ag jagttagelser, &c. Bergen, 1 835. 

 X Edinburgh New Philosophical Journal, 1835. 



§ For a clear and full abstract of the observations on tliis subject, sec Owen's 

 Lectures on the Invertebrata, p. '273. 



