ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 39 



crowded with Salpce, in all states of growth, and of a size very con- 

 venient for examination. At subsequent periods the writer had occa- 

 sion repeatedly to verify the results at which he had arrived, and to 

 find strong analogical confirmation in the structure of Pyrosovia and 

 other allied genera.^ 



4. It is proposed in t^ie following pages to consider — 



I. The structure of the species of Salpa examined. 

 II. The structure of Pj/r(?j'(?;««. 



III. The homology of structure oi Salpa and Pyrosovia, and of these 

 with the ordinary Ascidians. 



IV. The history of our knowledge of the Salpm. 



Section I. — The Structure of Salpa. 



5. Before entering upon this question, there is a point of some im- 

 portance to be determined, as to the upper and lower surfaces, the 

 anterior and posterior extremities of the Salpa. Observation will not 

 decide this apparently simple matter, for as the writer has frequently 

 seen, they swim indifferently with either end forward and with either 

 side uppermost ; and the determinations of authors are most contra- 

 dictory. 



Throughout the present paper, that side on which the heart is placed 

 will be considered as the dorsal side ; that on which the ganglion and 

 auditory vesicle are placed, as the ventral side. That extremity to 

 which the mouth is turned will again be considered as the anterior 

 extremity, the opposite as the posterior. Such a view of the case 

 appears to be more harmonious with the determinations of corres- 

 ponding parts in other animals than any other. In all the inverte- 

 brata the mouth end is always considered as the anterior, the heart 

 side as the dorsal side. 



6. The two so-called species of Salpa examined were the S. 

 democratica of Forskahl, spinosa of Otto, and the 5. inucronata of 



^ Those who are acquainted with the nature of the service on which H.M.S. Rattle- 

 snake was engaged, will readily comprehend that the author's investigations were almost 

 necessarily original, and independent of anything going on in Europe. 



It is the more necessary to state this, as it will be seen, in the historical part of this paper, 

 that M. Krohn, in the Ann. des Sciences for 1846, has completely anticipated the chief 

 results arrived at, a fact of which the author was totally unaware until his arrival in England 

 in the end of 1850. 



Still, as M. Krohn gives merely his conclusions without details or figures, his promised 

 memoir' not having appeared (so far as the writer of the present paper is aware), it is hoped 

 that this anticipation will, by showing that perfectly independent observers arrive at the 

 same result, rather tend to increase than to diminish any weight that may be attached to the 

 present researches. 



