670 



MOLLUSCA. 



special study, a;n3 wno nas adilea greatly to our l?nowleclge oP their organization, an-^ especially of 

 their development, into three orders, viz. — the Salpid/E, the Ascidiad^, and the Pyr05omid.e. 



r\r. I 



I. The Salpid.e are. in suiiil- particulars uf their or-anizatiun, tla- lii^Miest of tht Tunicata. They differ from the 

 truw Ascidians, as WLdl 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- 

 piratui'y currLiit i»-rsriill>- tn h<.' drsuribed. In form they rLSt.'inble short but ratli-.r wide tubes, "with an opening 



__P at each end ; the respiratory 



ai'puratus, instead of being 



J 1 \ the entire dilated 



here consists of 



li t 1 t branchial leaf- 



1 the branchial ori- 



IS f r -vided with a valve 



whi 1 p rmits the free en- 



t mec t water, but pre- 



1 nts its return. The vis. 



la ire collected into a 



s n^le mass at the posterior 



t\trcmity of the body ; and 



tl ma'^b IS very conspicu- 



u "ing to the brilliant 



, orange, brown, or reddish 



I orifice; f, lii-er, inclosing; otlicr ^ 



vj^cern; c, heart; ir, liraiichial snc ; m, muscular bands ; h, nervous gungliuii. hue of the liver, of ^vhich 



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

 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 Salpce, and the animals being perfectly free, they acQ^uire 

 a progressive movement through the water, the branchial orifice being directed forwards. The Salpre 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 adlaesion is less powerful, so that when a mass is placed in a vessel of water, the sides of 

 ivhich 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 

 Salpas produce chains of aggregate Salpm by a kind of internal gemmalion from a sort of strAon or creeping stem, 

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

 outside. These aggregate Salpffi 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- 

 ■\'iOuals of the chain of aggregate Salpte, eggs are produced, each of which developes itself into original form 

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

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

 by StL-enstrup and his followers. According to this doctrine, in those tribes in which such a scries of phenomena 

 is i.rrs;;nted as that just described, we are to interpret them as follows.— Generation A (e.g. a solitary Sa\p a,) 

 produces generation n, 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 oft'spring, 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 Salpai give origin to the 

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

 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, thut 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, tliat a new generation can be 

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

 hitherto possessed. We sliall have to return to this subject, when considering the curious relations which sub.sist 

 between certain Polyj'cs aud Mcduscc, in the Appendix to the Radiata. 



II. In the AsciDiAD^, the body is either fixed immediately to some solid mass, or is attached by a pednnele ; 

 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 famihes of 

 simple, social, and compound Ascidians. 



1. The Simple A scuHa.vs are completely detached from another 

 clusters, the individuals composing these h'lve no or-'anic uiiiuii 



for, alth 

 Theyge 



nigh rrei|LiL' 



it witli in groups ur 

 liv tn-al form. They 



