AND DEVELOPMENT OF PYROSOMA. 207 



The brancliial stigmata just described are subdivided into quadrate mesbes by some 

 fifteen longitudinal bars whicli lie altogetlier on the inner side of the vertical ones, to 

 which they are attached by their outer edges, projecting like so many narrow shelves into 

 the pharyngeal cavity, and, as I observed in my earlier memoir on this animal*, are devoid 

 of cilia. They terminate abruptly at their anterior and posterior ends, and they do not 

 exhibit the small denticulations along their free edges which I have described in F. atlan- 

 ticum. The water taken in by the oral aperture must pass with perfect ease through these 

 meshes, and then, impelled by the cilia on the vertical branchial bars, make its way 

 through the lateral atria and, on each side of the intestine, to the mid-atrium, whence it 

 finds an exit by the atrial aperture. 



As to the proper digestive canal, the wide aperture of the oesophagus lies at the posterior, 

 neural, angle of the pharyngo-branchial sac, and has an irregular figure ; Init whether this 

 irregularity is normal, or arises from the collapse of its walls after death, I cannot say. 

 The oesophagus narrows as it passes back, and then curves sharply round towards the 

 haemal side, to open, after a very short coui'se, into the large oval stomach, which lies im- 

 mediately behind the middle of the branchial sac, invested, everywhere but in front, by the 

 atrial tunic, and bathed in the blood which lies between it and that tunic. At its pyloric 

 end it gives rise to the narrow commencement of the intestine, which, after suddenly 

 dilating and turning forwards and to the haemal side, bends back sharply upon itself, and 

 passing backwards to the neural side and to the right, ends opposite the middle of the 

 stomach in the abruptly truncated anus, which opens into the atrium. 



In my memoirs on Salpa, Pyrosoma, and BoUoliim, already referred to, I have described, 

 in all these genera, a remarkable system of fine transparent tubes which ramify over the 

 intestine, and eventually open by a single duct into the stomach. I have asked (/. c. p. 570), 

 does this tubular system represent a hepatic organ ? or is it not more probably a sort of 

 rudimentary lacteal system — a means of straining off the nutritive juices from the stomach 

 into the blood by which these tubes are bathed ? In Pyrosoma giganteum the duct of the 

 system is very obvious, opening into the stomach in front of the origin of the intestine, 

 and somewhat enlarged at its opposite end. 



Krohn has described the structure and development of a similar system of tubuU in 

 Phallusiai, and I have since % found an organ of the same nature in Fhallusia, Cynthia, 

 Molgula, Perophora, Botryllus, Botrylloides, Clavelina, AjjUdium, Didemnum, and, in- 

 deed, in all genera of Ascidians which have come under my notice, except AppendiciUaria^ 

 In some species of Didemtmm, 1 have observed that the duct dilates almost at once into 

 a large spheroidal sac. I suspect that Savigny was the original discoverer of this system 

 (see his memoir on Diazona, I. c. p. 176, and the description of pi. 12). The existence of 

 these tubuli in Salpa, Doliohim, and Pyrosoma has been confirmed by all subsequent 

 observers. M. Vogt, however {I. c. p. 31), afiirms that the organ consists of solid branches, 

 and that it partakes of the nature of a muscular organ, in neither of which opinions can 

 I possibly concur. I have no doubt whatever that the apparatus is a glandular organ, 

 and that it performs a part, at any rate, of the functions of a liver. 



* Loc. cit. pi. 17. fig. 3, and p. .583, line 4, where the word ' sinus ' should be ' ones.' 

 f " Ueber die Entwickelung der Ascidien," Miiller's Archiv, 1852, p. 331. 

 J See Reports of the British Association, 1852. 



