2 THE VOYAGE OF H.M.S. CHALLENGER. 



In regard to the first of the above sections, the descriptive part of the Eeport, the 

 AscidiaB Salpiformes inchides the single family Pyrosomatidse, and forms the third 

 suborder of the order Ascidiacea, — the fii'st and second being the Ascidiae Simplices and 

 the Ascidise Composita?, — and so completes that group. The Thaliacea is a second 

 order, and includes the important genera Salpa and Doliolum, along with some less well- 

 known allied forms. The thu-d order, Larvacea, contains the Appendiculariidse, and 

 so completes the Tunicata. Consequently, although the forms discussed in this part of 

 the Report are all pelagic free-swimming Tunicates, still they are not closely related, 

 but fall into three distinct groups, which are less allied to one another than the 

 Ascidiee Salpiformes are to the Ascidiaj Simplices or to the Ascidiae Compositse. 



By far the greater number of the pelagic Tunicata collected by the Challenger 

 Expedition belong to the genus Salpa, and to a few of the more common and widely- 

 distributed species of that genus, so that a great part of the work has consisted in the 

 laborious examination of large numliers of specimens of a species from various localities 

 in order to determine whether they were all the same. The specimens were all in 

 rectified spirit, and were most of them well enough preserved for systematic and 

 anatomical purposes ; while some few were in an excellent state of preservation, and 

 aiforded material for the elucidation of some histological details. 



Altogether the collection of pelagic Tunicata contains at least twenty-six species, of 

 which nine are new to science. It has not been found necessary to establish any new 

 genera ; but a new family, the Octacnemidse, has been formed for the reception of the 

 remarkable deep-sea genus Octacnemus, Moseley. The plates illustrate the new species 

 and new structural details in regard to some of those already known. 



In the second section of this Eeport I have incorporated, with the sanction of Dr. 

 John Murray, those theoretical conclusions at which I have arrived in regard to the 

 relationships of the Tunicata and the course of their evolution, after nearly ten years' 

 continuous study of the group, in the course of which I have examined for myself 

 nearly every generic type, and by far the greater number of the known species. 



A small number of pelagic Tunicates, collected during the cruises of H.M.SS. 

 " Porcupine," " Knight Errant," and " Triton," which had been sent to me for 

 examination by Dr. Murray, are also described in this Eeport. 



The Appendices include — 



A. A description of a few Simple Ascidians sent to me since the publication of 



the last part of this Report. 



B. An account of a remarkable Dorsal Tubercle found in a fragment of a large 



species of Ascidia from Kerguelen Island. 



