OF THE SALP^. 47 



The Salpce have always been placed near the Ascidia, 

 and the more accurately the structure of both these 

 families is known, the more evident does their relation- 

 ship become. 



Eor this knowledge, we are recently indebted to the 

 last memoir* of Milne Edwards, on the compound Asci- 

 dia, or those which are regularly grouped in large masses 

 or colonies, {Ascidia composites.) This excellent natural- 

 ist has pointed out a peculiar circulation of the blood, 

 common to all the Ascidia, whether solitary {AscidicB 

 sinqjJices,) or in small groups {A. sociales, M. Edw.,) or 

 closely aggregated and for the most part regularly 

 grouped in larger masses or colonies {A. composit(B;) and 

 a similar circulation has also been discovered in I'yrosoma 

 which may be considered a swimming colony of ascidi- 

 cms, and in the 8alp(s.\ 



What, however, the more justifies us in delaying a 

 moment with the compound ascidians before quitting the 

 consideration of the series of the animal kingdom, to 



* Milne Edwards, Observations sur las Ascidies Composees des cotes de la 

 Manche, 1841. Avec planches, gr. 8vo. 



f Li this cii'culation the blood is carried from the heart, and back again to 

 it through the same vessels. The heart, which is always placed close to the 

 generative organs, and usually ia the inferior part of the animal, is moderately 

 extended, contracts with an undulating motion, and propels the blood in the 

 same direction with the undulations ; for instance, if these are directed from 

 below upwards, the blood ascends in a stream along the anterior surface of 

 the abdomen, within a wide thoracic or abdominal vessel, from which a 

 number of contiguous transverse canals proceed, which are connected by 

 munerous smaller longitudinal vessels, and form the four-sided vascular net- 

 work on the walls of the branchial sac. AH the transverse canals open into 

 a large dorsal vessel, which receives thus all the blood which has passed 

 through the vessels of the branchial sac ; but besides this some blood is also 

 received by it which has not passed through the respiratory organ, in conse- 

 quence of the great dorsal vessel being connected at each extremity with the 

 great thoracic or abdominal vessel, oy two considerable amiular vessels, 

 'riie blood returns downwards from the dorsal vessel through a canal lybg 

 on the dorsal surface of the abdomen (dorsal canal) to the opposite end of 

 the heart. After the lapse of a few minutes the heart begins to undulate in 

 the contrary du-ection, which is necessarily followed by the blood; which 

 now flows upwards during the same space of time through the dorsal canal 

 and dorsal vessel, traverses the branchial sac, and passing through the great 

 abdominal vessel along the anterior surface of the abdomen again reaches the 

 heart. In this way the blood flows alternately in opposite directions, accord- 

 ing to the direction of the movements of the heart. 



