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U)iiversif!j of Calif or)iia FuhJicafions in Zoologu [Vol. 18 



Although there are important structural differences between indi- 

 viduals of the two generations, they are superficially so nearly alike 

 that the following description, borrowed mainly from Brooks (1893, 

 p. 4), relates as well to an individual of one generation as to one of 

 the other: 



Each individual is transparent, ovate in form, and small, rarely 

 exceeding eight millimeters in length. Its body may be compared to 

 a barrel open at both ends so that water flows through it wdthout 

 obstruction (text fig. A). The mouth (m) occupies the anterior 

 end of the barrel, and the lips are infolded so as to act as valves per- 

 mitting the entrance of water but preventing its escape. At the 



Fig. A (after Eitter). Seniidiagrammatic representation of the solitary gen- 

 eration of Salpa democratica: (a) atrial aperture; (atr) atrium; (end) endo- 

 style; (g) gill; (m) mouth; (n) nucleus or intestinal tract; (ph) pharynx; 

 (st) portion of stolon which has been converted into individuals of the aggre- 

 gate generation; (t) test or mantle cavity. 



opposite end of the barrel the atrial aperture (a) affords an exit for 

 the water but, owing to a few sphincter muscles, prevents it from 

 entering. Essentially, the chamber of the barrel is uninterrupted 

 from mouth to atrial aperture for, though divided by a rod-like gill 

 (g) into the pharynx (ph) and the atrium (atr), the gill is so nar- 

 row that it offers little obstruction to the water, and there is a large 

 free passage on all sides of it. 



The body is partly encircled by six bands of muscles somewhat 

 like barrel hoops crowded together on the upper surface midway 

 between the two ends of the barrel, and spread apart on the lower 

 surface. Contraction of these muscles empties the barrel, driving 

 the water out of the atrial aperture, and propels the salpa through the 

 water in the opposite direction. The body is encased in a thick, trans- 

 parent mantle or "test" (t) which, by its elasticity, antagonizes the 

 muscles and draws in a fresh supply of water. "The animal there- 



