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



Suborder I. ASCIDI^ SIMPLICES.' 



This group contains fixed Ascidians which arc solitary, and very rarely reproduce 

 l)y gemmation ; if colonies are formed, the members of the colony are not buried in a 

 common investing mass, but each has a distinct test of its own. 



The Ascidise Simplices include four families. The Challenger Simple Ascidians 

 were described in Part I. of this Report, and in Part II. Appendix A.' 



Suborder II. ASCIDI^ COMPOSITE." 



This group contains fixed Ascidians which reproduce by gemmation so as to form 

 colonies in which the Ascidiozooids are buried in a common investing mass, and have 

 no separate tests. 



The Ascidise Compositae include seven families. The Challenger Compound 

 Ascidians were described in Part II. of this Report. 



Suborder III. ASCIDI.E SALPIFOEMES. 



This group contains free-swimming pelagic Ascidians which reproduce by gemmation 

 so as to produce colonies having the form of a hollow cylinder closed at one end. The 

 Ascidiozooids forming the colony are embedded in the common test in such a manner 

 that the branchial apertures all open on the outer surface and the atrial apertures on the 

 inner surface next to the central cavity of the colony. The first four Ascidiozooids are 

 produced by gemmation from a rudimentary larva (the cyathozooid) developed sexually. 



The Ascidia3 Salpiformes include a single family, the Pyrosomatid-E, containing one 

 well-marked genus Pyrosoma. 



Family Pyrosomatid.e, 



Colony free-swimming, and having the form of a hollow cylinder closed at 



one end. 

 Systems — only one present, the terminal aperture being the common cloacal 



opening. 

 Ascidiozooids elongated antero-posteriorly, and placed in a single layer with their 



branchial apertures opening on the surface of the colony and their atrial 



apertures into the axial common cloaca. Body not divided externally into 



regions. Apertures not lobed. 



1 See also Part I. of this Report (Zool. Chall. Exp., part xvi., 1882), page 57. 



' See also Appendix A. of the present Part. 



' See also Part II. of this Report (Zool. Chall. Exp., part xxxviii., 1886), page 28. 



