AGGREGATED ASCIDIANS. 489 



form the one to produce a new being, the other to fecundate a future 

 generation of animals similar in all respects to themselves. But whilst 

 the isolated Salpians are thus produced from eggs, their progeny are 

 produced by a process of gemmation, springing like buds from the sur- 

 face of a most remarkable organ, the stolon prolifer, the existence of 

 which is to be detected in the isolated Salpae even while contained in 

 the body of their concatenated parent ; it then appears a slender fila- 

 ment, derived by one extremity immediately from the heart of the 

 embryo Tunicary ; after birth, however, its growth increases apace in 

 proportion to the development of the continual succession of progeny to 

 which it gives origin. The reason of the immediate connexion between 

 the " stolon proliferum " and the maternal heart appears to be this, 

 that the newly-formed offspring being entirely dependent for support 

 upon the blood of the parent, it is so situated in order to secure a free 

 supply of the vital fluid, which is thus injected into its vessels imme- 

 diately by the heart's action. Two vessels traverse it throughout its entire 

 length, one derived from the anterior extremity of the maternal heart, 

 and the other from its opposite end; so that the blood supplied to one of 

 these vessels by the contraction of the heart is returned by the other ; 

 and when the contractions of the heart become reversed, as we have seen 

 is continually the case by this arrangement, the circulation in these vessels 

 is readily adapted to the change. It is from the surface of the stolon 

 that the generation of concatenated Salpae sprout, in two parallel rows, 

 appearing in rapid succession like so many little buds, which, as their 

 growth advances, gradually assume a similar form and structure ; and 

 as successive groups become mature, detaching themselves from the 

 nidus where they had their birth, they swim away united in long 

 chains, the links of which are joined together after the fashion of the 

 species. On examining one of these chains of concatenate Salpae, the 

 individuals composing it are found to be united to each, not by any 

 organic coalescence, but by special organs and facets of attachment, 

 frequently, but improperly, described as suckers, the position of which 

 varies in different species in accordance with their mode of aggregation. 

 It would appear, from the observations of M. Krohn, that the concate- 

 nated Salpas cannot spontaneously detach themselves from each other, 

 and that, when individuals are met with swimming free, their separation 

 from the chain is always to be ascribed to accidental violence ; he even 

 thinks that concatenation is so essential to the existence of the animals 

 that they soon perish if separated from the rest. 



(1285.) The last families of TUNTCATA which we have to notice would 

 seem to constitute a connecting link between the ASCIDIANS and the 

 POLYZOA, which latter in many points of their anatomy they much 

 resemble. These animals generally are exceedingly minute, and indi- 

 vidually present an organization analogous to that of Ascidians. At 

 first it would appear that they are detached from each other, and, like 



