ha. 
TUNICATA. 701 
Orver IT. Tethyonidea. 
Body sacciform, with two apertures mostly approximate. Cover- 
ing coriaceous or gelatinous, mostly opaque. Branchial sac large, 
reticulato-fenestrate, with rectangular areole; the beginning of 
cesophagus 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 Aleyonidia (see above, p. 78). 
The discovery of their true affinity is one of the most interesting 
results of the accurate investigations of Savicny. Compare on this 
order the above-cited works of this author, as well as those of MiLnE 
Epwarps, VAN BeNEDEN, &e. 
Besides the two divisions of Ascidice into simple and compound, 
Minne Epwarps has adopted a third, that of the social Ascidie, 
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', and may perhaps 
occur in all the animals of this division. 
Young Aseidians, 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 Mine Epwarps (Ann. des Se. nat. xv. p. 10), 
as early as 1828, and was afterwards more fully described by 
V. Benepen and others. They fix themselves by that extremity 
which is opposite to the tail, which they then lose. In compound 
Ascidians, according to the observations of Sars on Botryllus, such 
a cercarie-form 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 BoHADSOH in A scidia intestinalis (Phallusia intestinalis Sav.). See 
J. B. Bowapscu De quibusdam Animalibus marinis. Dresde, 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 Cercarie-form larve, consult R. Leuokart Ueber Morphologie und die Ver- 
wandtschaftsverhiltnisse der Wtrbellosen Thiere. Braunschweig, 848, 8vo, 8. 173, 174. 
