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



PL II. fig. 4. The Ascidiozooids are closelj' packed together, aud all lie iu the same 

 plane. Each is rudely triangular in shape, with the base external and the apex 

 towards the centre of the colony. Between the four blunt ajDices a sj)ace is left in 

 which lie the young buds. The branchial aperture of each Ascidiozooid opens upon a 

 Hat area of test, and between each pair of Ascidiozooids there is a similar flat area of 

 test ; thus the eight flattened sides of the colony are formed (PI. II. fig. 4). At 

 each of the angles of the octagon the test is prolonged outwards to form a pair of 

 closely placed triangular pointed spines. There is a low ridge of test surrounding each 

 of the four flattened areas upon which the branchial apertures of the Ascidiozooids 

 open, and from these ridges shorter triangular spines project upwards at intervals. 

 These are not shown in PI. II. fig. 4, which represents an oj^tical section. 



In surface views of the closed and open ends of the colony they are seen to diff'er in 

 shape and arrangement of spines l^oth from one another and from the middle of the 

 colony as seen in optical section. The closed end (PI. II. fig. 2) is rudely quadrangular 

 in shape, and is divided into four nearly equal parts by ridges springing from the 

 middle of the sides and meeting in the centre. Each of the four parts bears two 

 pointed projections on its outer end. These are unequal and irregular in shape, and 

 in one case only one spine is present. 



The open end of the colony (PI. II. fig. 3) is stellate in outline. It has eight points, 

 four of which are rather larger than the intermediate ones. These four are the outer 

 ends of the triangular areas (Aj to A4) in which the Ascidiozooids lie, while the four 

 smaller points are the outer ends of the regions of test lying between the Ascidio- 

 zooids. Eight ridges of test run inwards from the reentrant angles between these 

 points, and so divide this end of the colony into eight radially disposed bands. In 

 the centre is placed the common cloaca! oj^ening, which is still small. Its edge is 

 crenated, each of the eight ridges forming a projection. 



The Ascidiozooids have their ventral sides turned to the closed end of the colon}\ 

 The Ijranchial sac is large. Its anterior end is very wide, and it tapers to the narrow 

 posterior end (PI. II. fig. 4). The transverse vessels and internal longitudinal bars of 

 the branchial sac, the si^hincter muscle of the branchial aperture, the cndostyle, the 

 nerve-ganglion, and the alimentary canal, are all well formed ; but no reproductive 

 organs are visible. It is iuterestins; to find that the lara;e ventral tentacle is the 

 only one of the circlet that has made its appearance. 



At the posterior end of each Ascidiozooid is seen a small bud attached to a short 

 stolon springing from the body wall close to the posterior end of the endostyle (PI. II. 

 fig. 4). This colony is evidently very young, and contains only the first four 

 Ascidiozooids formed in the embryo. From the arrangement of the spines and 

 lidgcs on the test it probabl)' Ijclongs to the same species as the young colony to 

 be described next (No. 2). 



