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TUNICATA. 



must comprehend the two dissimilar states 

 which have hitherto been falsely considered as 

 two distinct species, and to which have been 

 given specific names, that can now serve only 

 to designate one or the other of the hetero- 

 morphous conditions.* 



Amongst the characters that distinguish 

 the different generations of each species of 

 Salpa, one of the most remarkable is offered 

 by the disposition of the muscular bands. 

 These are variable in the two generations, 

 but constant for each of them. There is, 

 however, a still more essential difference, and 

 that is, the mode of propagation peculiar to 

 each generation. The aggregate individuals 

 proceeding from the isolated individuals grow 

 by gemmation within the mother animals, on a 

 cylindrical prolongation, which may justly be 

 termed a proliferous j* stolon, but which 

 differs from stolons observed in many other 

 animals, in not ever appearing externally. 

 The stolons of the " social " Ascidians spring 

 up bare, the animal not being connected by a 

 common gelatinous envelope, as is the case 

 with the "compound" Ascidians; in which 

 latter group the stolons necessarily remain 

 hidden in the common envelope, somewhat 

 approaching in this respect the stolons of the 

 Salpians. 



The solitary Salpians derived from the 

 aggregate generation are, on the contrary, 

 produced by a process more complicated 

 namely, by means of the sexual functions, the 

 concurrence of the eggs and the sperm. 



Generation in the "aggregate" Salpians. 

 With very few exceptions, the individual 

 aggregate Salpians produce only one offspring 

 throughout their lifej, so that, if we examine 

 them at one period, anterior to fecundation, 

 we find a single egg, and, at a later period, a 

 foetus. The egg is distinctly visible within 

 the young aggregate individual before it 

 has left its parent (isolated) Salpian ; and 



as the fecundation of this egg takes place 

 immediately, or at least a very short time 

 after, the birth of the young aggregate Salpa, 

 it can be examined before fecundation, that 

 is, during the development of the aggregate 

 within the isolated individual, or shortly after 

 its birth. The egg is lodged in the thickness of 

 the internal tunic of the mother, at a little dis- 

 tance from the inner wall of the respiratory 

 cavity. It is very clearly distinguishable in the 

 aggregate embryo at a period intermediate be- 

 tween the appearance of the solitary embryo 

 as a bud and its full development. It is then 

 situated above the visceral mass, at the an- 

 terior extremity of the body, and nearly in the 

 middle line, raising the external tunic into a 

 slight prominence. It is spherical, and consists 

 of a vitellus, containing the germinal vesicle 

 and spot, and invested with a membrane so 

 thick, that M. Krohn is led to regard it as 

 comparable with the " calyx" in birds. During 

 the progress of the development of the ani- 

 mal, the position of the egg is altered, and it 

 becomes situated on the side of the body, 

 somewhat approaching towards its superior 

 surface, and behind the second muscular band. 

 This position is retained by the egg, and sub- 

 sequently by the foetus. 



To the posterior extremity of the egg a cord 

 is attached, which serves as a sort of peduncle, 

 in general directed nearly horizontally back- 

 wards, and consisting apparently of a pro- 

 longation of the membrane that covers the 

 vitellus. At the period when the egg oc- 

 cupies the anterior extremity of the embryo, 

 this cord is proportionally thicker and shorter 

 than at the subsequent periods of the em- 

 bryo's development. M. Krohn considered 

 it as being the nutritive peduncle of the 

 ovarian capsule, or membrane, enveloping the 

 egg. This cord exists, as above described, up 

 to the time of fecundation, but it soon after- 

 wards disappears. The ovary, we see, is 



* To the above observations, M. Krohn has added the following list of such species of Salpa as he had 

 been enabled to recognise in the two states : 



IN THE SOLITARY STATE (Proles solitaria, 



Chamisso). 



f Salpa democratica, Forskahl\ 

 \ S. spinosa, Otto y 



SPECIES. 



1. 



2. Salpa africana, Forskahl 



3. Salpa runcinata, Chamisso - 



4. Salpa, observed by Krohn - 

 f Salpa scutigera, Cuvier 



K JS. vivipara, Peron and Lesueur 

 * S S. gibba, Bosc 



(^S. dolium, Quay and Gaimard 

 6. Salpa, observed by Krohn 



IN THE AGGREGATED STATE (Proles gregata, 

 Chamisso). 



f Salpa mucronata, Forskahl. 

 \ S. pyramidalis, Quoy and Gaimard. 

 f Salpa maxima, Forskahl. 

 \ S. Forskahlii, Lesson. 

 C Salpa fusiformis, Cuvier. 

 < S. maxima, var. prima, Forskahl. 

 { S. runcinata, gregata, Chamisso. 

 Salpa punctata, Forskahl. 



f Salpa bicaudata, Quoy and Gaimard. 

 \ S. nephodea, Lesson. 



Salpa proboscidalis, Lesson. 

 f Salpa zonaria, Chamisso. 

 S. polycratica, Forskahl. 



I Salpa cordiformis, Quoy and Gaimard 



"S The recognition of the two forms of this species was made by Prof. Eschricht, and con- 

 C. firmed by M. Krohn. 

 8 f Salpa costata, Quoy and Gaimard \ ( Salpa Tilesii, Cuvier. 



( Dagysa strumosa, Banks j \ S. infundibulifonnis, Quoy and Gaimard. 



M. Krohn observes also, that Salpa ferruginea, Chamisso; S. confsederata, Forskahl; S. socia, Bosc; 

 S. octofora, Cuvier (?); S. laevis, Lesson, and S. femoralis, Quoy and Gaimard, do not materially differ 

 from S. bicaudata. 



j- Professor Eschricht was the first to recognise 

 the true signification of this part. 



J S. zonaria is the only example of an aggregate 

 Salpa producing several (four) foetuses. 



