TUNICATA. 



1233 



rapid, and the movements of the blood-discs 

 can be more easily followed. "As these blood- 

 globules are of a certain consistence, a resist- 

 ance," says Van Hasselt, " is necessarily cre- 

 ated in the whole mass of the blood, which 

 ultimately overcomes the projectile power of 

 the heart. After a short interval, during which 

 the opposite forces are balanced, the heart 

 assumes a spiral movement, contrary to 

 the preceding. Hence it follows," he ob- 

 serves, " that, since the blood is driven as 

 much backward as forward in the vessels of 

 the body, and since it is only by their anas- 

 tomoses that the circulation can be said to be 

 carried on, all the system of the pulmonary 

 vessels can consist only in accessory ramifica- 

 tions, which have no direct influence on the 

 principal circulation; and that two separate 

 systems, arterial and venous, do not exist, 

 the two being united, or rather never having 

 been separate."* 



The nerve- ganglion is more or less de- 

 veloped in all the Salpians. It is situated on 

 the superior surface of the thoracic chamber, 

 near the posterior extremity, and just above 

 the insertion of the branchia. In its neigh- 

 bourhood is often observed a pigmentary spot, 

 or rudimentary organ of vision. 



In S. cordiformis the ganglion appears as a 

 compound body, formed of two central glo- 

 bular portions and four irregular lateral ap- 

 pendages, two on a side. Numerous filaments 

 radiate off in every direction. Four of these 

 filaments are sent towards the posterior ori- 

 fice, two of them passing on either side of the 

 space described by Prof. Eschricht as the 

 " oval organ." This latter is a long slit with 

 an inner smooth rim, and an outer oval, trans- 

 versely striated border. In S. zonaria the gan- 

 glion is a somewhat triangular, globose body, 

 with a single appendix on each side, sending 

 off four anterior filaments, two lateral pairs 

 and a posterior pair. The " oval organ," situ- 

 ated between the last pair, is close up to the 

 ganglion, and less distinct than in S. cordiformis. 



Many of the Sulpce are highly phosphores- 

 cent \\ but sometimes this condition is due 

 to the presence of phosphorescent Crusta- 

 ceans inhabiting their internal cavity. The 

 long chains of phosphorescent Salpce swim- 

 ming near the surface, have been described 

 as occasionally producing the effect of long 

 ribands of fire drawn along by the currents. 



Generation of Sulpce. " One of the most 

 important discoveries," says M. KrohnJ , " with 

 which physiology has been enriched in our 

 days, is without contradiction that of the 



* Extrait d'un Lettre de Van Hasselt, sur les 

 Biphores, le 12 Aoiit, 1821 (Algem. Konst-en Let- 

 terbode, 1822). Annales des Sciences Naturelles, 

 torn. iii. 1824, p. 78. Bull, des Sciences, torn. ii. p. 212. 



f See Mr. G. Bennett's Observations on the 

 Phosphorescence of the Ocean, " Proceedings of the 

 Zoological Society, 1837 ; " and his " Wanderings 

 in New South Wales." See also the Article, LUMI- 

 KOUSXESS, ANIMAL. 



J Observations sur le Generation et la DeVeloppe- 

 ment des Biphores. Annales des Sciences Natu- 

 relles, 3 e series, torn. vi. 184G. 



VOL. IV. 



remarkable phenomena observed in many 

 inferior animals, and termed, * propagation by 

 alternate generations.'" To M. Steenstrup* 

 the merit is incontestably clue, of having been 

 the first to bring together, in an ingenious man- 

 ner, all the facts belonging to the subject. In 

 treating of the phenomena observed by Cha- 

 misso in one species of Salpa, and analogous 

 phenomena studied in other groups of animals, 

 and in his generalisations of the facts, M. Steen- 

 strup has placed them in a strong light, and 

 given them their full value. M. Steenstrup's 

 observations have also tended to strengthen the 

 views of Chamisso -J- so often contested ; and 

 the researches of M. Krohn, on the natural 

 history and zootomy of eight species of 

 Salpce, fully confirm them.J Every Salpian 

 is viviparous ; and each species, as shown by 

 Chamisso, is propagated by an alternate suc- 

 cession of dissimilar generations. One of 

 these generations is represented by solitary or 

 isolated individuals ; the other by aggregated 

 or associated individuals, united into groups 

 known as " chains." Each isolated individual 

 engenders a group of aggregated individuals, 

 and each of these produces in its turn a 

 solitary individual. The isolated individuals 

 are therefore multiparous, and the associated 

 individuals uniparous. This is not the only 

 difference existing between the two alternating 

 generations ; for, if we compare the associated 

 individuals, representing the aggregate gener- 

 ation, with the solitary individual forming the 

 isolated generation, we shall find that they 

 differ amongst themselves not only with 

 respect to external conformation, but also in 

 many particulars of organisation. 



The definition, then, of species should in 

 this case include the characteristics of the two 

 dissimilar generations, isolated and aggregate, 

 which alternately succeed each other. It is 

 desirable therefore, for the determination of 

 each species, to preserve but one name. This 



* Ueber den Generationswechsels in den Niedern 

 Thierklassen, 1842. 



f De Animalibus quibusdam a Classe Vermium 

 Linneana. Fasc. 1. "DeSalpa." Berolini, 1849, 4to. 



J M. Steenstrup, in his memoir on the Alternation 

 of Generation (edit. Roy. Soc. pp. 39 and following), 

 gives a full account of the different views of Chamisso, 

 Meyer, and Eschricht, with regard to the develop- 

 ment of the Salpce. He allots considerable space to 

 the consideration of the arguments of the learned 

 Danish professor, with whose opinions, however, he 

 does not coincide, acknowledging that all his own re- 

 searches tend to support the observations of Chamisso. 



M. Steenstrup notices that the alternate genera- 

 tions of Salpce are precisely analogous to the phe- 

 nomena observed in the propagation of Medusa 

 aurita. This free-swimming Medusa always pro- 

 duces a progeny which is polypiform and destitute 

 of the power of locomotion, but which ultimately 

 brings forth, by tissiparous generation, a progeny 

 consisting of free-swimming Medusae, which never 

 assume the polypoid form ; and this alternation is 

 constant. M. Sars observes, on the generation of 

 Salpce (Erichson's Archiv. 1841, t. i. p. 29.), that 

 " the Salpce correspond in this with the Medusa:, 

 that it is not their larvce which are developed into 

 the perfect animal, but the progeny of the larvce; it 

 is not the individual, but the generation, which is 

 metamorphosed." 



4 K 



