The embryonic chain, as we have seen, is lodged in the external tunic of the mother. During the earlier periods of its growth, the substance of the tunic envelops it so closely, that no interval is apparent between them ; but, as the mass of embryos increases in size, there is formed around it, and chiefly around the most fully developed group of embryos, a cavity which is prolonged towards the surface of the mother's body, and opens externally by a large orifice. By this orifice the embryos, when mature, make their exit. The posi tion of this opening always corresponds with the point where the embryonic chain happens to terminate, which is sometimes near the anterior extremity, but sometimes even at the posterior extremity of the body of the 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 Salpre 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" Salpa.— Each group of aggregate Salpre 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. piunata, 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 (S. maxima, S. fusffornzis, S. punctata, S. zonaria).* 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 node 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.