194 



TUNICATA 



Struc- This sub-order includes a single family, the PYEOSOMIDJB, con- 

 ture of taming one well-marked genus, Pyrosoma (Peron), with several 

 Pyro- species. They are found swimming near the surface of the sea, 

 soma. chiefly in tropical latitudes, and are brilliantly phosphorescent. 

 A fully developed Pyrosoma colony may be from an inch or two to 

 upwards of four feet in length. The shape of the colony is seen in 

 fig. 20. It tapers slightly towards the closed end, which is rounded. 

 The opening at the opposite end is reduced in size by the presence 

 of a membranous prolongation of the common test (fig. 20, B). 

 The branchial apertures of the ascidiozooids are placed upon short 

 papillae projecting from the general surface, and most of the ascidio- 

 zooids have long conical processes of the test projecting outwards 

 beyond their branchial apertures (figs. 20, 21, and 22). There is 

 only a single layer of ascidiozooids in the Pyrosoma colony, as all 

 the fully developed ascidiozooids are placed with their antero- 

 posterior axes at right angles to the surface and communicate by 

 their atrial apertures with the central cavity of the colony (fig. 21). 



Fio. 21. Part of a longitudinal section through wall of Pyrotoma, showing 

 arrangement of ascidiozooids, magnified (partly after Savigny). at, atrial 

 apertures ; br, branchial apertures ; asc, young ascidiozooid of a future colony 

 produced by budding from cy, cyathozooid ; em, embryos in various stages ; 

 t, test ; tp, processes of test ; brs, branchial sac ; yas, young ascidiozooid. 



Their dorsal surfaces are turned towards the open end of the colony. 

 The more important points in the structure of the ascidiozooid of 

 Pyrosoma are shown 

 in fig. 22. A circle of 

 tentacles, of which 

 one, placed ventrally 

 (fig. 22, in), is larger 

 than the rest, is 

 found just inside the 

 branchial aperture. 

 From this point a f 

 wide cavity, with a 

 few circularly-placed 

 muscle bands run- 

 ning round its walls, 

 leads back to the 

 large branchial sac, 

 which occupies the 

 greater part of the 

 body. The stigmata 

 are elongated trans- 

 versely and crossed 

 by internal longitu- 

 dinal bars. The dor- 

 sal lamina is repre- 

 sented by a series of 

 eight languets (I). 

 The nerve ganglion 

 (on which is placed 

 a small pigmented f 

 sense organ), the sub- 

 neural gland, the dor- 

 sal tubercle, the peri- 

 pharyngeal bands, 

 and the endostyle are 



placed in the usual 



<"> , i Fio. 22. Mature ascidiozooid of Pyrosoma, from left 

 positions. On each 6id e (partly after Keferstein). Lettering as before ; 

 side ot the anterior cm, cellular mass, the seat of phosphorescence; 

 end of the branchial c "*'. posterior cellular mass ; gs, gemmiparous 

 sar close to thp npri stolon ; mb, muscle band ; ngl, subneural gland ; 



pharigeal bands, 7s *" pigment 8p0t a ganglion : * process f test ' 

 a mass of rounded gland cells which are the source of the phosphores- 

 cence. The alimentary canal is placed posteriorly to the branchial 

 eac, and the anus opens into a large peribranchial (or atrial) cavity, 

 of which only the median posterior part is shown (pbr) in fig. 22. 

 The reproductive organs are developed iu a diverticulum of the peri- 



branchial cavity, and consist of a lobed testis and a single ovum at 

 a time. The development takes place in a part of the peribranchial Deve] 

 cavity (fig. 21, tm). The segmentation is meroblastic, and anment 

 elongated embryo is formed on the surface of a mass of yolk. The Pyro- 

 embryo, after the formation of an alimentary cavity, a tubular soma, 

 nervous system, and a pair of laterally placed atrial tubes, divides 

 into an anterior and a posterior part. The anterior part then 

 segments into four pieces, which afterwards develop into the first 

 ascidiozooids of the colony, while the posterior part remains in a 

 rudimentary condition, and was called by Huxley the "cyatho- 

 zooid " ; it eventually atrophies. As the four ascidiozooids increase 

 in size, they grow round the cyathozooid and soon encircle it (fig. 21, 

 asc and cy). The cyathozooid absorbs the nourishing yolk upon 

 which it lies, and distributes it to the aseidiozooids by means of a 

 heart and system of vessels which have been meanwhile formed. 

 When the cyathozooid atrophies and is absorbed, its original atrial 

 aperture remains and deepens to become the central cavity of the 

 young colony, which now consists of four ascidiozooids placed in a 

 ring, around where the cyathozooid was, and enveloped in a common 

 test. The colony gradually increases by the formation of buds from 

 these four original ascidiozooids. 



PHYLOGENY, 



The accompanying diagram shows graphically the pro- Phyl 

 bable origin and course of evolution of the various groups 

 of Tunicata, and therefore exhibits their relations to one 

 another much more correctly than any system of linear 

 classification can do. The ancestral Proto-Tunicata are 

 here regarded l as an offshoot from the Proto-Chordata 

 the common ancestors of the Tunicata (Urochorda), Am- 



POLYST-., 



POLYCL. 



phioxus (Cephalochorda), and the Vertebrata. The ances- 

 tral Tunicata were probably free-swimming forms, not 

 very unlike the existing Append iculariidx, and are repre- 

 sented in the life-history of nearly all sections of the 

 Tunicata by the tailed larval stage. The Larvacea are the 

 first offshoot from the ancestral forms which gave rise to 

 the two lines of descendants, the Proto-Thaliacea and the 

 Proto-Ascidiacea. The Proto-Thaliacea then split into the 

 ancestors of the existing Cydomyaria and Hemimyaria. 

 The Proto-Ascidiacea gave up their pelagic mode of life 

 and became fixed. This ancestral process is repeated at 

 the present day when the free-swimming larva of the 

 Simple and Compound Ascidians becomes attached. The 

 Proto-Ascidiacea, after the change, are probably most 

 nearly represented by the existing genus Clavelina. They 

 have given rise directly or indirectly to the various groups 

 of Simple and Compound Ascidians and the Pyrosomidx. 

 These groups form two lines,, which appear to have diverged 

 close to the position of the family Clavelinidx. The 

 one line leads to the more typical Compound Ascidians, 

 and includes the Polyclintdse, Distomidx, Didemnidx, 

 Diplosomidx, Ccelocormidse, and finally the Ascidiee Sal- 

 piformes. The second line gave rise to the Simple 

 Ascidians, and to the Botryllidx and Polystyelidx, which 



1 By Dohrn and others their point of origin is placed considerably 

 further up on the stem of the Chordata, thus causing the Tunicata 

 to be regarded as very degenerate Vertebrata (see 32). 



