W. K. BROOKS ON THE GENUS SALPA. 87 



dorsal surfaces outwards, and their long axes parallel to the axis of the 

 wheel. In all my preserved specimens the tip of the stolon had been so 

 much flattened by contact with the side of the bottle, in transportation, 

 that I have not been able to study in detail the way in which this 

 wheel-like arrangement is acquired, and the subject should receive the 

 attention of those who are able to study living specimens. 



There is an obvious resemblance between the wheel-like arrange- 

 ment of the first four ascidiozooids which in Pyrosoma form the basis 

 for the cylindrical community, and the wheel-like aggregation of the 

 Salp in species of the pinnata group, as may be seen by comparing 

 Salensky's figures of the young Pyrosoma community (17, Taf. II) with 

 Plate XLI, Fig. 9. In each case the animals are arranged in a circle 

 with their long axes parallel to the central axis, their dorsal surfaces out- 

 ward and their oral ends above. I shall show that there are other 

 reasons for believing that the pinnata-like species of Salpa are most 

 primitive and most closely related to Pyrosoma, and it is not improbable 

 that the wheel-like arrangement has been inherited in both cases from a 

 common source, and that it is the primitive arrangement for the Salpae. 



In most species, however, the aggregated salpae are set free in 

 the well-known floating clusters which have long been called chains. 

 Part of a chain of Salpa cordiformis is shown in Plate IV, Fig. 6, and 

 part of one of Salpa scutigera is shown in Plate IV, Fig. 1. As is shown 

 in these figures, a chain consists of two parallel longitudinal rows of 

 individuals, so placed that those in one row alternate with those in the 

 other, while the neural or dorsal surfaces of all are external, and their 

 ventral surfaces in contact with the ventral surfaces of those on the 

 other side of the chain. 



The members of the community are united to each other by process 

 from the walls of their bodies, which are hollow and contain diverticula 

 from their body cavities, although there is no communication between 

 the body cavities of adjacent salpae. 



In the pinnata group, Salpa pinnata, Plate I, Fig. 3 ; Salpa chamis- 

 sonis, Plate VIII, Fig. 6 ; Salpa affinis and Salpa dolichosoma, there is 

 only one of these processes, situated on the middle line of the ventral 

 surface in front of the heart. The way in which it arises is well shown 

 in the figures in Plate VII, Figs. 4 and 5, and Plate VIII, Fig. 1. Plate I, 

 Fig. 3, shows it in Salpa pinnata in its perfect form, while the other 

 figures show it at earlier stages. It is also shown in section, at succes- 

 sive stages, in Plate XXXVI, Fig. 9, Plate XXXVII, Figs. 10 and 21, 

 and Plate XXXVIII, Figs. 52 and 61. 



