384 



Div. 2. MOLLUSCA.— TUNICATA. 



special study, arid wlio lias added greatly to our knowledge of their organization, anfi especially of 

 tlieir development, into three orders, viz. — the SALPiDiE, the AsciDiAD^. and the Pyuosomid^. 



I. The Salpid* are, in somepiirticulars of their organization, the highest of tlie Tuuicata. They differ from the 

 true Ascidians, as well iu 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 iu clusters ; their movement being due to the res- 

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



at each end ; the respiratory 

 apparatus, instead of being 

 formed by the entire dUated 

 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 brilUant 

 orange, brown, or reddish 



Fig 6. 



-BiPHORA, ONE OF THE SaI.P^; o, 



viscera 



mouth i />, posterior or anal orifice ; /i liver, inclosing other 



, he:trt; 6)-, hralichial sac ^ jn, nmscular hands ; n, nervous ganglion. hUC 01 the livcr, of whlch 



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 Salpce, arid the animals being perfectly free, they acquire 

 a progressive movement through the water, the branchial orifice being directed forwards. The Salpce 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 Salpa by a kind of internal gemination from a sort of stolon or creeping stem, 

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

 outside. These aggregate SalpK 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 solitary fia.\\ya,,) 

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

 resemble generation A ; so that each individual is unlike its own immediate parents and offspring, 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 Salpce 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 Salpa;, the individuals thus budded off" become completely detached from their stock, and cxliibit 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 sh.-Ul have to return to this subject, when considering the curious relations which subsist 

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



II. In the AsciDiADJE, 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 compound Ascidians. 



1. The Simple Ascidums 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 



