W. K. BROOKS ON THE GENUS SALPA. 89 



species which have just been named this inclined arrangement is per- 

 sistent. 



In still other species, such as Salpa cordiformis, Plate IV, Fig. 6, 

 Salpa runcinata, Salpa africana and Salpa hexagona, the axes of the 

 bodies rotate until they become nearly or quite horizontal and parallel to 

 the axis of the chain, and we thus have two series of salpae with each one 

 joined at its anterior or oral end to the posterior end of the one next in 

 front of it in the same row, and with the two rows facing each other by 

 their ventral surfaces, and with the middle of the body of each opposite 

 the joint where two in the opposite row are joined together. 



While the union between the salpa3 in the series is ectodermal, the 

 cellulose mantles, as they grow, usually come into contact, and flattening 

 against each other to help to maintain the integrity of the chain. In 

 chains like those of Salpa scutigera, Plate IV, Fig. 1, and Salpa demo- 

 cratica, Plate XLIII, Fig. 1, the cellulose mantle is in contact, on the 

 sides of the body, with that of adjacent salpae in the same row, while in 

 all chains the ventral surfaces of the mantles of the salpa3 on opposite 

 sides of the chain come into contact. In chains like that of Salpa cordi- 

 formis, Plate IV, Fig. 6, where the salpae in each row are placed end to 

 end, the area of contact between their mantles is increased, as is shown 

 in the figure, by pyramidal processes at the ends of the body. As long 

 as the chain is intact these processes are bent at right angles to the long 

 axis, of the body of the salpa, but when the chains are broken up by 

 storms or other accidents the processes gradually straighten out into the 

 long axis of the body, as is shown in Plate III, Figs. 2 and 3, which are 

 dorsal and lateral views of a detached specimen of the aggregated form 

 of Salpa cordiformis. 



This brief sketch of the characteristics of salpa chains should be 

 followed by a discussion of their comparative history and of the phylogeny 

 of the chain, but it will be best to postpone this for the present and to 

 treat it in a separate section together with other questions of phylogeny, 

 and we are now prepared to study the details of the process by which the 

 stolon becomes converted into a series of salpae. 



SECTION 3. The Segmentation of the Stolon. 



The statement that the stolon becomes converted into, or is mor- 

 phologically equivalent to, a single row of salpae placed like a file of 

 soldiers with their long axes vertical or at right angles to the stolon and 



