18 THE VOYAGE OF H. M.S. 'CHALLENGER. 



The general shape of a Pyrosoma colony is seen in fig. 1. Small colonies are as a 

 rule more regular, and taper more towards tlie closed end than large ones. The narrower 

 (tlosed end is rounded, the other is truncated, and has a larger or smaller opening in 

 its centre (PL I. figs. 1, 4). The size of this aperture, or rather of the lip or diaphragm 

 which reduces it, has been used as a character distinguishing species ; but F. D. Bennet * 

 showed, in 1837, that in the living Pyrosoma the diaphragm can be moved so as to 

 allow the aperture to enlarge and contract (see below, p. 28). Fig. 1, B, shows the open 

 end of a Pyrosoma colony in which the diaphragm is extended so as to leave only a 

 small central aperture. 



The processes projecting from the sui"face of the colony vary considerably in arrange- 

 ment, size, and shape in different species and colonies. Each one indicates the anterior 

 end of an Ascidiozooid, and the branchial apertures can be seen (Fig. 1) either at the 

 ends, or at the bases, or half way up the processes. The usual arrangement is for each 

 large Ascidiozooid to form a dome-like projection or pajoilla on the surface of the colony 

 at the extremity of which the branchial aperture is placed, while a longer or shorter 

 process of the test extends outwards beyond that (see Figs. 2 and 4). 



taivx: 9^"^- 



y.asc. %gg 



Fig, 2. — Part of a section through a Pyrosoma colony. 



at. atrial aperture, al.m. atrial muscle, hr. branchial aperture, hr.s. brancliial sac, cm. young embiyo, cm!, older 

 embryo, sliowing cyathozooid and ascidiozooids, en. endostyle, gem, bud ou stolon, ij.st. gemmiparous stolon, 

 I. dorsal lanquet, ??.;/. nerve ganglion, a', cesophagus, ov. ovum, p.hr. peribranchial cavity, st. stomach, (. test, 

 Us. testis, t.p. process of test, tn. tentacles, y.asc. young ascidiozooid, z. zona prebranchialis. 



The Ascidiozooids are placed in a single layer, each having, when fully developed, 

 its branchial aperture opening on the outer surface of the colony, and its atrial aperture 

 opening into the central cavity of the colony or common cloaca. Consequently the 

 antero-posterior axis of the Ascidiozooid coincides with the thickness of the wall of the 



' Proc. Zool. Soc. Land, part v. p. 51. 



