1190 



TUNICATA. 



rous, slightly prominent, annular or subellip- 

 tical, without central cavities; the animals 

 (3 to 25) are placed in a single row, at 



SigilUna australis. (After Savigny.} 



A. Vertical section of a cone, or " system" showing 

 the distribution of the animals, magnified. 



B. Isolated individual, magnified. 



c. Natural size of isolated individual. 



a, test, or external envelope ; b, branchial orifice ; 

 c, anal oriSce ; d, thorax ; e, abdomen ; f, oeso- 

 phagus ; g, stomach ; h, intestine ; j, thoracic sinus ; 

 k, intestinal loop ; /, ovary ; m, oviduct. 



equal distances from their common centre ; 

 branchial orifice six-rayed ; division of the 

 thorax and abdomen not always distinctly 

 marked. 



Genus Sidnyum, Savigny. Common body 

 gelatinous, presenting a series of closely 

 grouped cones, truncated and starred at the 

 summit, rising from a common incrusting base ; 

 each cone composed of a fascicle of indi- 

 viduals, varying in number from five or six 

 to ten or twelve, and forming a margin around 

 a depressed centre. The animals partake 

 of the characters of those of Synoicitm and 

 ApKdium, resembling the former in the struc- 

 ture of their stomach, and the latter in their 

 branchial sac. Each has an eight-toothed 

 branchial orifice and a simple tubular vent 



folded against the thorax. The ovary is jpe- 

 duncled, and very conspicuous at the ex- 

 tremity of the animal. (Forbes.) 



Genus Synoicum, Phipps.* Common body 

 semicartilaginous, cylindrical, pecluncled, iso- 

 lated or attached to similar cylindrical bodies ; 

 system single, circular, comprising six to 

 nine animals, terminal; branchial orifice six- 

 rayed; anal orifice having six very unequal 

 rays, the three largest forming the exterior 

 margin of the central star ; post-abdomen 

 sessile. 



Genus Amaroucium, Milne-Edwards. Com- 

 mon body fleshy or coriaceous, polymorphous, 

 subpedunculate or sessile, and incrusting ; 

 systems numerous ; individuals arranged irre- 

 gularly around common cloacal apertures ; 

 divisions of the thorax and abdomina faintly 

 marked. This genus resembles Aplidium and 

 Synoicum in the general form of the animal, 

 and Polyclinum in the presence of a central 

 common cavity to each system (Jig. 782 ). 



The tribe Didemnina (Its Didennriens, Milne- 

 Edwards) is formed of all the compound 

 Ascidians whose body is distinctly divided 

 into two parts, thorax and abdomen. These 

 closely approach the ClavcUince, and are dis- 

 tinguished from the Polydinina by the absence 

 of the post-abdomen and by the position of 

 the organs of generation and the heart, which 

 ..are raised up by the side of the intestine. 

 This tribe, like the Polycimma, is divided into 

 two groups, according to the presence or ab- 

 sence of marginal rays around the anai orifice. 

 M. Milne-Edwards observes that the bistellate 

 group (Di&tomus and Diazona} forms a con- 

 . necting link between the Clavcllinee and the 

 BotryllidcB. 



Genus JDistomus, Gaertnert ; synonyms, 

 Alcyonium, Auct., Polyzona, Fleming. Com- 

 . mon body semi-cartilaginous, polymorphous, 

 sessile ; systems numerous, usually circular; 

 individuals placed in one or two ranks at un- 

 equal distances from their common centre. 

 Both orifices six-rayed. 



Genus Diazona, Savigny. Common body 

 gelatinous, orbicular, sessile or subpedunculate ; 

 system single, expanded into a disc, compara- 

 ble to that of a flower or of an Actinia; 

 animals very prominent, arranged in con- 

 centric circles (fig. 770.) ; branchial and anal 

 orifices six-rayed ; abdomen peduncled ; 

 ovary enclosed in the intestinal loop. 



Genus Didewnum, Savigny. Common body 

 coriaceous, polymorphous, sessile and incrust- 

 ing ; systems numerous, compressed, without 

 central cavities or distinct circumscription ; 

 animals without any appreciable order of 

 arrangement; abdomen peduncled; ovary 

 placed by the side of the intestinal loop, in- 

 creasing in length when the eggs are fully 

 developed. 



Genus Euccclium, Savigny. Common body 

 gelatinous, sessile and incrusting ; systems 

 numerous, without central cavities or distinct 



* Voyage towards the North Pole, 1773, p. 199, 

 pi. 13. 



f Gaertner, apud Pallas, Spicilegia Zoologica, 

 1774. 



