1238 



TUN 1C ATA. 



mother. At the moment of the birth of a 

 set of embryos, they detach themselves from 

 that part of the stolon which supported them, 

 and it then withers and disappears. 



The embryos, as we have seen, are placed 

 along the stolon in such a manner, that the 

 axes of their bodies cross the axis of the 

 stolon at right angles. This original position 

 of the individual either persists from birth 

 throughout life, or is changed, accordingly as 

 the new-born animals belong to one or other 

 of the three types of aggregate form to which 

 the associated Salpes are hereafter referred. 

 In the associated Salpce of the first type, the 

 relative position of the individuals remains 

 such as it was at birth. In the Salpians of 

 the third type, however, a change of posi- 

 tion of the young individuals is very mani- 

 fest, for the form of the generality of these 

 Salpians is not quite perfected at birth. The 

 two pyramidal processes with which the bodies 

 of S. maxima and S.fusiformis are furnished, 

 are but slightly developed in the newly born 

 animals. The growth of these prolongations 

 during youth is accompanied with marked 

 changes in the position of the respective in- 

 dividuals of the group. When these processes 

 are but small, the body of the animal is slightly 

 inclined to the axis of the chain. This direc- 

 tion becomes the more oblique as the two 

 prolongations increase ; and lastly, when they 

 have attained their full growth, the animal is 

 perfectly parallel to the axis of the chain ; and 

 now the development of the "aggregate" 

 Salpian is accomplished. 



Mode of arrangement and attachment of adult 

 "aggregate" Salpce. Each group of aggregate 

 Salpce is composed of a greater or less number 

 of individuals of the same size. Sometimes 

 the individuals are grouped in a simple cir- 

 cular series around a common axis, as in S. 

 pinnata, and some allied species. Sometimes 

 the individuals are arranged one after another 

 in two longitudinal, parallel series, and so dis- 

 posed that the individuals of one series alter- 

 nate with those of the opposite series (fig. 

 772. c). The biserial aggregation presents 

 numerous variations, according to the diver- 

 sities and forms peculiar to the associated 

 individuals of different species. In these 

 diversified modes of assemblage, M. Krohn 

 points out three types, to which all the va- 

 riations are reducible. The first is charac- 

 terised by the vertical position of the animals 

 forming the chain, so that the axes of their 

 bodies cross the axis of the chain at a right 

 angle (Salpa bicaudata, S.ferruginea). In the 

 second type the bodies of the individuals are 

 more or less inclined to the axis of the chain 

 (S. mucronata, S. Tilesii). The third group is 

 distinguished by the horizontal position of 

 the component animals, the axis of their 

 bodies being more or less parallel to the 

 axis of the chain (5. maxima, S. fusiformis, 

 S. punctata, S. zonaria).* 



* Meyen also made an analogous distribution of 

 the aggregate Salpians. Vide Nova Acta Nat. Cur. 

 torn. xvi. " Supplementum." 



In each group the individuals are in such 

 close approximation, that there exists no in- 

 terval between them, and the entire group 

 appears as one mass. The individuals touch 

 one another by the inferior surface, and more 

 or less also by the lateral surfaces of their 

 bodies. The superior surface, that where the 

 nervous ganglion is situated, and the two 

 orifices, remain perfectly free. Thus, for ex- 

 ample, in a chain in which the individuals are 

 disposed vertically or obliquely to its axis, the 

 members of each rank are united to those of 

 the opposite rank by their inferior surfaces, 

 and to their collateral neighbours by their 

 lateral surfaces. But, however closely the 

 individuals may adhere one to another, mutual 

 contact of the surfaces would not have been 

 sufficient to maintain their juxtaposition, had 

 not other appliances been furnished. There 

 are sometimes appendages of considerable 

 size, sometimes small protuberances, or only 

 circumscribed points of the surface of the 

 body, by the aid of which the animals adhere 

 among themselves so strongly, that they can 

 rarely be separated without some effort. These 

 special organs and facets of attachment have 

 been very incorrectly regarded by some authors 

 as suckers. Their number varies according 

 to the mode of aggregation. The associated 

 individuals of Salpa pinnata and allied species, 

 grouped in a circle, are provided with only a 

 single large appendage, springing from the in- 

 ferior surface of the body, and resembling 

 sometimes a crest (S. pinnata), sometimes a 

 horn (S. proboscidalis) . The individuals, few 

 in number, are united by the extremities of 

 these appendages, which meet one another 

 in the centre of the group. The organs 

 of attachment of Salpce aggregated in chains 

 are protuberances and facets, generally about 

 eight in number. Four are placed in pairs 

 on the inferior aspect of each animal, and 

 serve to unite it to its two neighbours in 

 the opposite series ; of the two other pairs, 

 one pair occupies one of the lateral faces, 

 the other the opposite lateral surface, uniting 

 the individual to its two collateral neighbours. 

 The position of these organs varies accord- 

 ing to the form of the associated individuals, 

 and accordingly as they belong to one or the 

 other of the above mentioned types. Of these 

 appendages we may notice the two prolonga- 

 tions of the body common in most of the 

 aggregate forms of the third type, as in S. 

 maxima, S.fusiformis, and chiefly by means of 

 which the contact of the individuals is pre- 

 served. Both are pyramidal in form, and arise 

 one from the anterior and the other from the 

 posterior extremity of the body. 



We will here remark that an individual 

 cannot spontaneously separate itself from the 

 group of which it forms a part. It is true 

 that free individuals are often met with, but 

 their separation is always due to some acci- 

 dent. M. Krohn thinks even that the union 

 in groups is so necessary to the maintenance 

 of the life of each animal, that it soon perishes 

 if by chance it becomes detached. 



