16 THE VOYAGE OF H.M.S. CHALLENGER. 



Saviguy's ^ account of the anatomy, witli the exception of the reproductive organs 

 and a few points of secondary importance, is very accurate and detailed. 



Lamarck/ in 1816, formed the class Tunicata and ranged the genus Pyrosoma, now 

 removed from the Mollusca, in the same order with the Compound Ascidians, and next 

 after Botryllus. He briefly characterises the genus and the three known species, and 

 appends a few remarks, but adds nothing to the anatomical discoveries of Lesueur and 

 Savigny. 



Milne-Edwards,' in 1840, investigated the circulation in Pyrosoma. He showed 

 that the nattire of the heart and the general course of the blood was much the same as 

 in the ordinary Ascidians. 



The next memoir of importance is Huxley's excellent Observations upon the 

 Anatomy and Physiology of Scdpa and Pyrosoma* published in 1851. His investi- 

 gations were made upon a specimen of Pyrosoma atlanticum captured in the middle of 

 the 8outh Pacific Ocean during the voyage of the " Rattlesnake." Huxley gives a very 

 complete account of the structure of this species, correcting some of the errors of his 

 predecessors, especially in regard to the reproductive organs. He showed for the first 

 time the true nature and position of the testis and ovary. In a very interesting section 

 of his paper on the Homology of Organs in the Tunicata, he shows the relationship of 

 Pyrosoma to Salpa on the one hand, and to the Botryllidse and the Compound 

 Ascidians on the other. Savigny^ had long before noticed the close resemblance 

 between Pyrosoma and Botryllus in some respects, and as I have already pointed out," 

 and shall discuss again more in detail farther on, I believe that the Ascidise Salpiformes 

 are much more closely related to the Ascidise Compositae than to any of the Tlialiacea. 



In a further paper On the Anatomy and Development of Pyrosoma, read in 

 December 1859, and published in 1862,' Huxley gave an elaborate account of the 

 structure, of the budding, and of the embryonic development of Pyrosoma giganteum, 

 from a specimen obtained in the North Atlantic, and preserved in spirit, and containing 

 many embryos in various stages of development. He was able to show that cross- 

 fertilisation must take place, since when the ova are mature in an Ascidiozooid, the testis 

 is still in a rudimentary condition. He traced the embryonic development, and the 

 formation of the remarkable Cyathozooid and of the series of four Ascidiozooids ** 

 attached to it, and their gradual conversion into the young colony. This was the first 

 accurate and consecutive account of the life-history of Pyrosoma; and it is an 



' Systeme des Ascidies, p. 205. 



* Histoire Naturelle des ADimaux sans Vertebres, torn. iii. p. 109. Paris 1810. 



^ Comptes i-endus, torn. x. p. 284 ; also A/in. Sci. Nat. (Zool.), scr. 2, torn. xii. p. 375. 



* Phil. Trans. IM.iil, part ii. p. 5G7. ^ Memoires. 

 " In Part II. of this Itoport, published in vol. xiv., 1886. 



" Tran.i. Linii. Sue. LniiiL, vol. xxiii. p. 193 ; also Ann. and Mag. Kal. Hist., 1860, ser. 3, vol. v. pp. 29-35. 



* This useful term was first introduced by Hu.'cley in his memoir in the 1'rans. Linn. Soc. Lond. {Joe. eh. supra). 

 In the prt'limiuary paper in the Ann. and Mci'j. Xat. Hist. (1860), he uses the term " Ascidiite." 



