670 



MOLLUSCA. 



special study, and wno nas added greatly to our icnowiedge of their organization, and especially of 

 their development, into three orders, viz. — the Salpid^, the Ascidiad^, and the Pyrosomid.5;. 



I. The SALPiD.e are, in some particulars of their organization, the highest of the Tunicata. They differ from the 

 true Ascidians, as well in their habits as in their structure. They are not attached to solid bodies, but habitually 

 swim in the waters of the ocean, sometimes singly, sometimes in clusters ; their movement being due to the res- 

 piratory current presently to be described. In form they resemble short but rather wide tubes, with an opening 



at each end ; the respiratory 

 apparatus, instead of being 

 formed by the entire dilated 

 pharynx, here consists of 

 two distinct branchial leaf- 

 lets, and the branchial ori- 

 fice is provided with a valve 

 which permits the free en- 

 trance of water, but pre. 

 vents its return. The vis. 

 cera are collected into a 

 single mass at the posterior 

 extremity of the body ; and 

 this mass is very conspicu. 

 ous, owing to the brilliant 

 orange, brown, or reddish 

 hue of the liver, of which 



losing othe 



FlO 6.— BiPHOBA, ONE OF THE Sai.p^; a, mouth; p, posterior or anal orifice ;/, liver, i 

 viscera ; c, heart; *r, branchial sac ; m, muscular hands ; n, nervous ganglion. 



a large part of it is composed, and to the transparency of the remainder of the body. As in the other Tunicata, 

 a continual stream of water is drawn in through the branchial orifice by the vibration of the cilia with which 

 the respiratory and digestive surfaces are clothed ; and a current as constant is ejected from the anal orifice. 

 These two orifices being opposite to each other in the Salpcc, and the animals being perfectly free, they acquire 

 a progressive movement through the water, the branchial orifice being directed forwards. The Salpcc are met 

 with in two states, solitary and aggregated. The latter are simply adherent to each other by little suckers, not 

 being organically united like the compound and social Ascidians. The adhesion, however, is so strong in some 

 species, that it is easier to tear the bodies of the animals than to separate them from each other ; in other 

 species, however, the adhesion is less powerful, so that when a mass is placed in a vessel of water, the sides of 

 which are smartly struck, the individuals fall asunder. The curious observations of Chamisso, mentioned in the 

 text (p. 382,) have been fully confirmed, especially by the researches of Krohn ; who has further shown that 

 there exists in these animals, as in the Ascidians, a double mode of propagation. He found that the solitary 

 Salpae produce chains of aggregate Salpoe by a kind of internal gemmation from a sort of stolon or creeping stem, 

 resembling that of the social Ascidians, (Fig. 7,) but contained witliin their bodies, instead of extending on the 

 outside. These aggregate Salpae differ from the solitary individual from which they have sprung, in several 

 points of their conformation, so that they have been described as distinct species. But from the several indi- 

 viduals of the chain of aggregate Salpae, eggs are produced, each of which developes itself into original form 

 of the solitary Salpae. According to Krohn, every species of Salpa thus exists under two dissimilar forms ; and 

 on this fact, and others of a similar nature, the doctrine of an " alternation of generations" has been built up 

 by Steenstrup and his followers. According to this doctrine, in those tribes in which such a series of phenomena 

 is presented as that just described, we are to interpret them as follows. — Generation A {e.g. a soJitary Salpa, ) 

 produces generation B, a being of a different form (e. g. an aggregate Salpa) ; and the offspring of generation b 

 resemble generation a ; so that each individual is unlike its own immediate parents and ofispring, but resembles 

 its grandparents and grandchildren. In this statement, however, the fact is too much lost sight of, that the two 

 modes of reproduction alternate, as well as the two forms produced. The solitary Salpae give origin to the 

 aggregate forms, not by the sexual process and the development of ova, but by gemmation ; so that the relation 

 of the two is in reality the same, as that of the several individuals of the social and aggregate Ascidians (pre- 

 sently to be described) to the original founder of the colony ; the only essential difference being, that in the case 

 of the Salpae, the individuals thus budded off become completely detached from their stock, and exhibit a 

 difference of organization adapted to the difference of life which they are to lead. It is only when the sexual 

 process intervenes, and ova are developed from which new stocks originate, that a new generation can be 

 properly said to commence, unless we give to the term generation a much wider acceptation than it has 

 hitherto possessed. We shall have to return to this subject, when considering the curious relations which subsist 

 between certain Polypes and Medusm, in the Appendix to the Radiata. 



11. In the Ascidiad;e, the body is either fixed immediately to some solid mass, or is attached by a peduncle ; 

 the two orifices of the mantle are usually near each other (Fig. 8) ; the greater part of the internal cavity is 

 occupied by the branchial sac, which may be regarded as a dilated pharynx ; and the viscera occupy a compara- 

 tively small space at the bottom of this sac. (See Fig. 7.) This order may be divided into the three families of 

 simple, social, and compomid Ascidians. 



1. The Simple Ascidians are completely detached from another ; for, although frequently met with in groups or 

 clusters, the individuals composing these have no organic union. They generally approach the oval form. They 



