TUNICATA. 



1227 



pying the common mass, of which it becomes 

 a new inhabitant. The communication be- 

 tween the mother and the young animal be- 

 comes obliterated ; but for some time yet, all 

 the young individuals growing from the same 

 branch remain united by their pedicle, and it 

 is this union, apparently, that determines their 

 mode of grouping into " systems." * In JJi- 

 demnum gelatinosum, the buds growing on 

 these proliferous stolons are very different 

 in appearance from the ova expelled by 

 the animals ; for not only did they differ in 

 aspect and form, but their bulk is at first 

 twenty or thirty times less than that of the 

 vitelline mass of the ova. In the Amaroucium 

 proliferum, Milne-Edwards has frequently 

 found on the surface of a rounded mass, formed 

 by a colony of these animals, many little 

 filiform twigs, simple or branched, formed by 

 a prolongation of the common tegumentary 

 substance, and consisting of a tube closed at 

 the end, and enclosing, in its interior, one or 

 more embryos in different states of develop- 

 ment. These young individuals terminated 

 inferiorly by a peduncle, prolonged in form of 

 a slender tube into the common mass, and 

 springing apparently from the abdominal tunic 

 of an adult individual. This mode of propa- 

 gation by buds, which the compound Ascidians 

 possess in common with the Polypi/era, is, as 

 we have above described, found in the Clavel~ 

 linidce ; the only important difference being, 

 that in the latter the tegumentary envelope of 

 the young is not so largely developed as in 

 the JSotryllidfS, and does not become fused 

 with that of the adults; whence it results that 

 the individuals springing from the same stem 

 remain isolated, instead of being united into 

 a common mass. 



ANATOMY OF PVROSOMA. The common 

 tegumentary mass of Pyrosoma is semitrans- 

 parent, subcartilaginous, toughish, and some- 

 what extensile. The exterior of the hollow, 

 conical, or cylindrical body, formed by the ag- 

 gregation of the individual Pyrosomata, is co- 

 vered with numerous elongated tubercles, of a 

 rather firmer consistence than the rest of the 

 mass. Each of these constitutes one extremity 

 of an individual member of the living group. 

 The opposite extremity opens into the cavity of 

 the cylinder, and is not free, but, like the trunk 

 of the individual's body, is closely connected 

 by the common mass with the similar parts of 

 other individuals lying above, below, and on 



* M. Savigny figures a nascent system, originated 

 apparently by this grouping of the buds ; and 

 Professor Van Beneden coincides with M. Milne- 

 Edwards in the above view of the subject, but M. 

 Steenstrup expresses his opinion that the mode in 

 which the colonies and systems of the BotryllidcB are 

 formed, is not sufficiently explained by this hypo- 

 thesis; and, although (following Milne-Edwards) 

 he considers M. Sars to have been misled in re- 

 garding the ova of the Botrylli as producing 

 groups of animals, yet he is inclined to consider this 

 grouping to be really a foetal condition, but occur- 

 ring in some hitherto unnoticed " aggregate " form 

 of animal, produced from the "solitary" larvae de- 

 scribed by Milne-Edwards, just as the sectary Salpce 

 bring forth Salpa-chains. 



either side of it. In some species the animals 

 are arranged much more regularly than in 

 others, and appear to form piled-up rings or 

 circles of individuals, more or less analogous 

 to the otherwise disposed circular systems of 

 some of the Botryllidce. In Pyrosoma 



ticitm the tubercles are simply conical, and 

 are perforated terminally. In P. elegans, also, 

 the external orifice of the individual opens at 

 the extremity of the tubercle, and through it 

 the water contained in the great cylinder has 

 been seen to escape freely in little jets, when 

 the Pyrosome has been taken out of the sea. 

 In P. giganteum the tubercles are of various 

 sizes, some being short and indistinct, and 

 others, on the contrary, very much deve- 

 loped. The largest are conico-cylindrical flat, 

 and lanceolate at the extremity, with the 

 minute branchial orifice on the inferior aspect. 

 This lanceolate extremity is crenulated on its 

 sharp edges, and presents on its inferior aspect, 

 between its point and the aperture, a slightly 

 prominent keel. The branchial orifice is 

 sometimes surrounded by a slight, free, crenu- 

 lated membrane. 



The interior of the great cavity is generally 

 smooth. Its walls are perforated by the 

 numerous minute anal orifices of the com- 

 ponent individuals ; and, at a slight depth, its 

 surface is studded with a great number of 

 yellowish, rose-coloured, or carmine spots, 

 which are the hepatic and other visceral 

 organs of the numerous animals. The ter- 

 minal aperture of the large, conical, compound 

 body of the Pyrosoma has, according to Le- 

 sueur and Savigny, a membranous border, 

 which can be sometimes drawn together so as 

 to close the cavity ; and Mr. F. D. Bennett 

 observed that, when first removed from the 

 sea, the broader extremity of the cylinder pre- 

 sented a wide and circular orifice, forming 

 nearly a continuous surface with the central 

 tube ; but when the animal was kept in a 

 vessel of sea-water, or much handled, this 

 orifice was closed by the contraction of a 

 smooth, dense membrane at its margin, and 

 which either obliterated the aperture, or left 

 but a minute central orifice; water at the 

 same time being contained in the barrel or 

 tube of the body. 



Besides the common envelope or test, each 

 individual animal has an inner tunic or man- 

 tle. This is a very thin, delicate membrane, 

 attached apparently at four points only, two 

 of which are at the extremities; that is, at the 

 branchial and anal orifices ; and the other two 

 are at two rounded, compressed bodies, one 

 on either side, just beyond the anterior mar- 

 gin of the branchiae, and regarded by Savigny 

 as ovaries. 



The branchifB line the inner surface of this 

 inner tunic. They are oval in form, and their 

 dorsal borders meet each other, and are at- 

 tached along the dorsal aspect of the mantle ; 

 but they are separated, at their anterior and 

 ventral borders, by a considerable space, which 

 is partly occupied by the ventral sinus (Jig. 

 786. i, i). The branchial tissue is traversed 

 by numerous vessels anastomosing with each 



