14 
THE VOYAGE OF H.M.S. CHALLEHGEK. 
shape; — round, fusiform, and stellate or branched shapes are the most common (PL XI. 
fig. 1, and many other figures). They comparatively rarely become modified into 
bladder cells or show much pigmentation. Bladder cells, if present, are usually in the 
outer layers of the test, and are of rather small size compared with those of Simple 
Ascidians. 
A coloured test may be due to either of two causes — (l) the presence of numerous 
vessels containing coloured blood-corpuscles, (2) pigment cells in the test. The vessels 
are usually most abundant at the edges of the colony and close to the surface, while the 
pigment cells may be scattered evenly all through the test, or aggregated in one part, as 
in the case of Colella murrayi (see p. 117). In some cases {e.g., many Didemnidee, 
PL XLI. figs. 2, 5) calcareous spicules ^ are formed in the test, and these may become 
Fig. 2. — A diagrammatic section through part of the mantle and test, showing their relations and the mode of 
formation of a “ vessel.” 
bl.c, a bladder cell in the test ; e, the ectoderm ; m, the mantle ; m.c, a connective tissue cell in the mantle ; t.c, test 
cells ; i.m, the matrix of the test ; s, a blood sinus in the mantle ; s’, its continuation into the test to form a 
“ vessel ” ; y, the septum of the young vessel formed by a continuation of the connective tissue of the mantle. 
so numerous as to entirely change the character of the tissue, rendering it hard and 
Ijrittle, and usually giving it an opaque white colour (see PL XXXV. fig. 1). 
As in the case of the Ascidise Simplices, the test is sometimes penetrated by “ vessels ” 
in the form of prolongations from the mantle covered by a layer of ectoderm and con- 
taining a blood sinus (see fig. 2). These vessels may be very numerous, and have large 
terminal knobs or bulbs placed in the superficial layers of the test (as in some Botryllidse), 
or they may be only very slightly developed (as in some Polyclinidse). In some cases they 
contain powerful bands of muscle-fibres continuous with those of the mantle, and then 
seem to have changed their function (which was probably respiratory at first ^ ) and to 
’ The relation of the.se spicules to the rest of the test is discussed further at p. 270. 
^ See Ilerdman, On a New Organ of Respiration in the Tunicata, Proc. Lit. and Phil. Soc. Liverpool^ 1884-85, p. 39. 
