EEPOET OX THE THXICATA. 
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or they may proliferate, in the neighbourhood of the Ascidiozooids, to form the groups of 
closely -placed cells with large nuclei. These cells then deposit between them a calcareous 
spicule, around which they are, when it is completed, stretched in the form of a thin 
membranous capsule containing nuclei. Such a capsule may be readily stained in the 
case of most of the large discoid spicules, and is sometimes seen hanging loosely from 
fragments of broken spicules. The nuclei are rery like those of the rounded cells in 
figure 8, and intermediate conditions may be found covering the smaller spicules. 
The calcareous capsule around the Ascidiozooid is formed of a large number of circular 
disks placed vertically in the layer of test immediately surrounding it (PI. XIX. fig. 3, S2^.). 
The disks are in most parts so numerous as to overlap one another’s edges (PI. XIX. 
fig. 7), and are on an average 0'4 mm. in diameter. Each disk is thickest in the centre, 
and tapers towards the circumference. This, as well as the general arrangement, may be 
seen from the transverse section of an Ascidiozooid and the neighbouring part of the 
common test shown in figure 3. It is also seen that the capsule is by no means a 
regular uniform covering, but is thick in some places (where it may be five or six spicules 
deep), and thin or absent in others. Some of the disks are not perfectly flat, but are 
slightly cuix^ed, as seen in section, and are placed with the concavity towards the body 
of the Ascidiozooid. In the cases of a few of the spicules in the figure (PI. XIX. fig. 3, sp.), 
the capsule is seen to fit very loosely, leaving a large open space between itself and 
the enclosed spicule. Whether this is natural, or a result of the method of ^^reservation 
and after-treatment, it is difficult to say. The disks, when seen in surface view as in 
figures 6 and 7, are beautifully marked by delicate concentric bands and by radiating 
lines. The latter in many cases go in groups, being absent or more openly placed 
in particular regions of the spicule, and closer in others (PL XIX. fig 6). 
The mantle is of considerable strength. The musculature over the greater part of it 
is in the form of a close network formed by strong bundles of longitudinal and transverse 
bands running at right angles, and forming narrow quadrangular meshes (PI. XIX. fig. 9). 
When these muscle bands reach the anterior end they become the radiating and circular 
bundles which surround the branchial aperture in a somewhat irregular manner (PI. XIX. 
figs. 10, 11). The branchial sphincter is well developed and compact. Beyond it for a 
short distance the radiating muscle fibres are not collected into bundles, but form an 
evenly distributed layer which passes outwards, crossing several series of circular fibres. 
Further out, however, the radiating fibres become grouped together into definite bundles 
separated by clear spaces free from muscle fibres (PL XIX. fig. 10), and these bundles 
are continued down the sides of the body, as the longitudinal bands of the network shown 
in figure 9. The muscular fibres throughout the greater part of the mantle are very 
narrow, but in some places fibres of considerable size, up to 0'004 mm. in breadth, are 
met with. There are many variously shaped connective tissue cells with very large 
distinct nuclei scattered through the mantle (PL XIX. fig. 10). 
