484 



LECTURE XX. 



185 



The only conspicuous vital action in the Salpians is the rhythmical 

 contraction and expansion of the mantle ; in which the elasticity of 

 the outer tunic antagonises the muscular contraction of the inner one. 

 During expansion, the sea-water enters by the aperture (a), and is ex- 

 pelled, in contraction, by the opposite one (i), its exit by the first aper- 

 ture being prevented by the valve. The re-action of the jet, which 

 is commonly forced out of a contracted tube, occasions a retrograde 

 movement of the animal. The currents which successively traverse 

 the interior of the animal renew the oxygenated medium upon the 

 surface of the respiratory organ, bring the nutrient molecules within 

 the reach of the prehensile subspiral labial membrane of the mouth, 

 and expel the excrements and the generative products. Thus, a single 

 act of muscular contraction is made subservient, by the admirable co- 

 adjustment of the different organs, to the performance of the func- 

 tions of locomotion, nutrition, respiration, excretion, and generation. 

 The aggregate Salpians quit their gemmiparous parent asso- 

 ciated together in long chains. In Jig. 185, a, h indicates part of 



the first set of young Salpae : 

 c, d'rs, the second set : e,f\s, the 

 third set: ^is the stem with its 

 ^/^^ \ \ \ \ I I / / Z^ germs: /i shows the anterior ori- 



fices : ii are the visceral masses; 

 j j, the ganglia ; k k, the pos- 

 terior orifice ; /, the vessels ; and 

 *• m, the muscular band of the 

 branchial sacs. After floating 

 for a certain time, each indi- 

 vidual, as Dr. Chamisso first 

 discovered, propagates a young 

 one like itself. The solitary 

 Salpa propagated by each in- 

 dividual of the chain, is the 

 product of an impregnated ovum, and is, for a time, suspended by a pe- 

 duncle from the dorsal wall of the visceral cavity of the parent. In the 

 Salpa zonaria, however, as many as three of such pedunculated young 

 have been found in one parent. "When liberated, the solitary Salpa 

 grows to the size of the grand-parent, and then brings forth a social chain 

 of young Salpae, which, by the exercise of their uniparous generation, 

 again give origin to the solitary and multiparous individuals. Thus, ob- 

 serves Chamisso, only the alternate generations resemble each other. 

 The case is strictly analogous to the generation of the compound 

 Ascidians, of which the solitary young gives origin, by gemmation, to 

 a connected group, each individual of which again procreates, by 

 impregnated ovn, free individuals. 



Aggregated young of Salpa zonaria. 



