W. K. BROOKS ON THE GENUS SALPA. 297 



hand salpa in contact with each other on the middle line but not yet 

 united. These figures show how easily the study of transverse sections 

 of the stolon at right angles to these might lead to the erroneous belief 

 that the median atrium of a right-hand salpa is formed from the left 

 vesicle alone, and that of a left-hand salpa from the right one alone. 

 Plate VI, Fig. 1, is a proximal view of a right-hand salpa constructed 

 from a series of sections at a stage a little younger than those last 

 referred to. The right perithoracic vesicle is shown at g, the left at h, 

 and the atrial prolongation of the left at g'". The corresponding portion 

 of the right one points towards the eye of the observer, and is, of course, 

 indistinguishable. Fig. 2 of Plate VI shows the proximal surface of two 

 salpae, a right-hand one and a left-hand one, after the median atrium is 

 formed ; g in each is the right perithoracic vesicle, and h the left one, 

 while g'" in the right-hand salpa is the portion of the median atrium 

 which is derived from the left vesicle, h; and g'" in the left-hand salpa is 

 the portion derived from the right vesicle, g. It is clear from the descrip- 

 tion that the middle line of the median atrium is in each salpa near the 

 outer edge of the figure, over gr, in the right-hand salpa, and over h in the 

 left one. The rotation of the body soon carries the whole of the atrium 

 out on to the external surface, as is shown at g'" in Plate VII, Fig. 4, and 

 Plate XXXVI, Figs. 4, 5, 6 and 7; and it unites with the ectoderm at g v 

 on the middle line of the dorsal surface, and acquires a secondary opening 

 to the exterior at g\ Plate VII, Figs. 88 and 97. 



Seeliger's account of the perithoracic structures of Salpa democratica 

 (11), p.p. 18, 48 and 63, serves to show how difficult the study of a simple 

 structure may be made by a slight change of position, for phenomena 

 which can be observed with ease in Salpa pinnata are so obscure in Salpa 

 democratica that all the industry and technical skill which Seeliger has 

 devoted to this species have had very little outcome. His account of the 

 history of the perithoracic system is essentially as follows : The peritho- 

 racic tubes, which he calls the Seitenstrange, are mesodermal in their 

 origin, and are specialized out of a mass of mesoderm cells which give 

 rise also to the nerve tube of the stolon and to the genital rod. This 

 mesoderm passes into the stolon from the body of the embryo in an 

 unspecialized condition, and gradually becomes differentiated into these 

 organs after the stolon is formed. The folds in the ectoderm of the 

 stolon divide the " Seitenstrange " into a series of solid masses at the sides 

 of the stolon between the ectoderm and the endoderm. These bodies are 



