276 LECTURE XX. 



tends towards the opposite aperture, and terminates freely in the com- 

 mon cavity of the mantle. A single narrow plicated ribband-shaped 

 branchia extends obliquely lengthwise across the pallial cavity. The 

 heart is elongated, and in some species slightly curved and sacculated ; 

 it communicates with a large vessel at each extremity, one of which is 

 ramified principally upon the visceral mass ; the other upon the 

 branchia and the muscular tunics. 



From recent observations made by Dr. Edwards on young Pyro- 

 somata (a compound genus of Salpians), it appears that the circu- 

 lating currents change their direction periodically, by virtue of 

 peristaltic and antiperistaltic vermicular contractions of the heart, as 

 in the Ascidians. 



The sexes are distinct in the Salpians, as in the solitary Ascidians. 

 The ovarium or testis is usually of an oblong form, sometimes single, 

 sometimes double, adherent to the inner surface of the mantle ; where, 

 likewise, the embryo is developed. 



The only conspicuous vital action in the Salpians is the rythmical 

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

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

 expansion the sea-water enters by the posterior aperture, and is ex- 

 pelled, in contraction, by the anterior one ; its exit by the opposite 

 end being prevented by a valve. The reaction 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 

 functions of locomotion, nutrition, respiration, excretion, and genera- 

 tion. 



Certain genera of Salpians, as the phosphorescent Pyrosoma, are 

 permanently aggregated into a compound organic whole having a 

 definite form. All Salpians quit their viviparous parent associated 

 together in long chains ; after floating for a certain time under this 

 form the society is dissolved, and each individual, according to Dr. 

 Chamisso, propagates a solitary young one like itself. This 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, 

 observes Chamisso, only the alternate generations resemble each other. 

 The case is strictly analogous to the generation of the compound 



