TUNICATA. 705 



the name of Ascidium (Natuurk. Uitspanningen, i. bl. 97), and 

 LINNAEUS, in the twelfth edition of his Systema Naturae, changed 

 this name, without any reason that I am aware of, into Ascidia. 



These Ascidice or bag-pipes (zakpijpen*), as BASTER names them, 

 are always attached to other bodies, to rocks, shells, crabs, &c. Often 

 several individuals are united in a single group ; they never, how- 

 ever, form such a compound body as the preceding genera, which 

 are distinguished by the orificia analia being always turned towards 

 each other, and more or less really united, (SAVIGNY, op. cit. p. 120), 

 whilst the external covering is common to all the individuals that 

 combine to form the group. 



These animals ingurge water through the branchial aperture, and 

 eject it chiefly by the same aperture in jets, which may serve as 

 a defensive means for chasing animals away that attack them. 

 CUVIER asserts that the expulsion of water can be performed 

 through the branchial aperture alone. Those writers, however, who 

 have observed them alive, are unanimous in their testimony that the 

 ejection of water is effected through both apertures. CARUS tells, 

 that in a large specimen of Ascidia microcosmus he saw an opening 

 furnished with a membranous valve, which appeared to lead from 

 the branchial sac to the porus analis. Other writers, however, do 

 not speak of such an opening. On the supposition of LISTER and 

 MILNE EDWARDS that the branchial sac is perforated like a sieve 

 (see above, p. 693), the matter may be explained without difficulty. 

 Whether Ascidians also ingurge water by the cloacal aperture after 

 a vacuum in the gill-sac has been caused by contraction, as SAVIGNY 

 suspected (pp. cit. p. 100), deserves further investigation. 



Ascidians live on small organic particles, which are brought with 

 the water into the respiratory sac and thence to the (esophagus that 

 opens at its bottom. Sometimes, indeed, small crustaceans have 

 been found in the sac, but they would seem to have arrived there 

 fortuitously; for when they have been ingurged by an Ascidian they 

 are rather hurtful than beneficial, and in some cases even injure the 

 tissue of the gills. 



EYSENHARDT has published observations from which it appears that the 

 body of Ascidians in a singular manner may change into a formless mass, 

 on which other Ascidjans attach themselves, and take root. Nov. Act. 

 Acad. Cces. Leop. Carol. Vol. xi. 1823, pp. -249 272. 



Comp. also on these animals (besides the works of CUVIER, SAVIGNY and 

 VAN BENEDEN already cited) CARUS Beitrdge zur Anatomic und Physiologic 

 der Seescheiden, in MECKEL'S Arch. f. die Physiol. n. 1816, s. 569 590 



VOL. I. 45 



