TUNICATA. 701 



ORDER II. TetJiyonidea. 



Body sacciform, with two apertures mostly approximate. Cover- 

 ing coriaceous or gelatinous, mostly opaque. Branchial sac large, 

 reticulato-fenestrate, with rectangular areolse; the beginning of 

 oesophagus situated at the bottom of this sac. Animals mostly 

 affixed. Propagation oviparous and gemmiparous. 



To this order some compound animals belong which were formerly 

 referred, for the most part, to the Alcyonidia (see above, p. 78). 

 The discovery of their true affinity is one of the most interesting 

 results of the accurate investigations of SAVIGNY. Compare on this 

 order the above-cited works of this author, as well as those of MILNE 

 EDWARDS, VAN BENEDEN, &c. 



Besides the two divisions of Ascidice into simple and compound, 

 MILNE EDWARDS has adopted a third, that of the social Ascidice, 

 which are distinguished by forming gems, without being grown 

 together like the compound Ascidians. This gemmation has, how- 

 ever, been noticed in a species ordinarily simple 1 , and may perhaps 

 occur in all the animals of this division. 



Young Ascidians, which do not originate in gemmation but 

 proceed from eggs, undergo an interesting metamorphosis. In the 

 early stage they move freely, and are provided with a long tail, as 

 was communicated by MILNE EDWARDS (Ann. des Sc. nat. xv. p. 10), 

 as early as 1828, and was afterwards more fully described by 

 Y. BENEDEN and others. They fix themselves by that extremity 

 which is opposite to the tail, which they then lose 2 . In compound 

 Ascidians, according to the observations of SARS on Botrylhtg, such 

 a cercarise-forni larva may already enclose a group (eight) of united 

 Ascidians. Thus, even before the Ascidia has become attached, by 

 the division of the gem the commencement is made of a colony 

 which is capable of further multiplication by the formation of 

 gems. Not, however, in all compound Ascidians is this original 



1 According to BoHADSCH in Ascidia, intestinalis (Phallusia intestinalis SAV.). See 

 J. B. BOHADSCH De quibusdam Animalibus marinis. Dresdse, 1761, 4to, pp. 132 

 135, Tab. x. fig. 5. 



2 As to the question, which is the anterior and which the posterior extremity of 

 these Cercarise-form larvae, consult R. LEUCKA.ET Ueber Morphologic und die Ver- 

 wandtschqftsverhaltnisse der Wirbellosen Thiere. Braunschweig, 1848, 8vo, s. 173, 174. 



