188 THE ACEPHALA. § 172. 
‘pletely surrounds the body and has only an oral and anal opening; ® and 
with the compound species, it is continuous with the common substance which 
contains the individuals and binds them into more or less regular groups, 
and is, therefore, analogous to.a corallum, With the Lamellibranchia, and 
Brachiopoda, it is more or less open, or even maybe wholly divided into 
halves; © it has here the property, especially upon its borders, of secreting 
calcareous matter for the formation of the shell. 
§ 172. 
With the Tunicata, the mantle is remarkable both for its histological 
structure, and its chemical composition. Recent investigations have shown 
‘that, with the Ascidiae and Salpinae, it is composed of Cellulose and there- 
fore of a non-azotized substance.” 
Its anatomical structure is quite complicated. ‘Usually it can easily be 
separated into two or three layers, the internal one of which is composed, in 
some species, of a lamellated epithelium formed of a single layer of poly- 
gonal nucleated cells. 
Its principal mass in both the compound:and simple forms of this order, 
is formed of a single, or a double confluent layer of .a homogeneous trans- 
parent substance, through which are scattered granules, nuclei, groups of 
pigment molecules, cells, fibres, and crystals of carbonate of lime, — all 
varying according to genera and species, and often differently arranged in 
one and the same species. 
ranged in the inner portion of this 
2 These openings are properly only simple ori- 
fices of the cavity of the body, and correspond to the 
respiratory tubes of certain Lamellibranchia ; see 
below, § 190. 
3 With Mya, Panopaea, Pholas, Teredo, As- 
pergilium, the mantle is almost entirely closed, 
but it has two long fissures ‘at each extrem- 
ity with Solen, Cyclas, Tellina, Mytilus, Litho- 
domus and others; with the Ostracea, Pectinea, 
Arcacea, Naiades, and Brachiopoda, it is entirely 
open. 
1 This important fact was first stated by Carl 
Schmidt (Zur. vergleich. Physiol. d. wirbellosen 
Thiere. 1845, p. 61), with Cynthia mamillaris, 
and has subsequently been confirmed by Lowig 
and Kélliker, after the must careful investigations 
upon the entire order of Tunicata (Compt. rend. 
1846, p. 38). These two authors found this non- 
azotized substance, particularly in the different spe- 
cies of Phallusia, Cynthia, Clavelina, Diazona, 
Botryllus, Didemnum, Aplidium, Salpa, and Py- 
rosoma; but not with the other Mollusca, nor 
with the Annelides, the Helminthes, the Echino- 
dermata, the Acalephae, and the Polypi. It is 
certainly wanting in the true Infusoria, for Frus- 
tulia salina, which Carl Schmidt cites as be- 
longing to this order and as containing cellulose, 
is evidently a vegetable. Lowig and Koélliker 
justly fear, moreover, that this discovery will be 
quickly seized by those who deny that there.is any 
limit between the animal and vegetable kingdom 
(see loc. cit. p. 8). They seek, therefore, to oppose 
this view by insisting upon the circumstance that 
this cellulose is never found in a pure state in the 
mantle of the Tunicata, but always combined with 
other substances, and that, mereover, no animal 
has as yet been found entirely composed of this 
substance. 
2 Phallusia mamillarzs, sulcata, Cynthia papil- 
lata, pomaria, and Salpa bicaudata. 
But in each species, they are variously ar- 
mantle-substance.” In some species 
h 
and 
8 This b: it sul is Ng ) 
has the same chemical properties as cellulose. 
4 Kolliker has made very detailed investiga- 
tions upon the structure of this mantle. He has 
kindly allowed me to communicate his results, and 
authorized me to make use of them without waiting 
for the publication of his work in common with 
‘Léwig (Ueber das Vorkommen von Holzfaser im 
Thierreich), According to them, the middle layer 
of the mantle of Phallusia monachus, and sul- 
-eata, Clavelina lepadiformis, and <Aplidium 
gibbulosum, contains numerous nuclei and star- 
like crystals lodged in a transparent structureless 
substance. But the external layer of this organ is 
filled with very large round cells with very thin 
walls, containing no nucleus, but filled with a 
transparent liquid. With Clavelina lepadifor- 
mis, the peduncle and branches of the whole man- 
tle are so crowded with non-nucleated cells, som 
round and others el ted, that the b t 
substance is apparently absent. It has therefore 
quite the aspect of a vegetable tissue. With 
Aplidium gibbulosum, and Botryllus violaceus, 
the cells of the external layer contain carbonate of 
lime which ultimately so increases that it gives 
them a petrified aspect. With Didemnum candi- 
“dum, these petrified cells have calcigerous rays 
and are so numerous that the whole mass of this 
compound Ascidian appears filled with white star- 
- like corpuscles. 
_According to Milne Edwards, this is true also 
of Leptoclinum maculosum (Observ. sur les As- 
cidiés composées, p. 81, Pl. VIII. fig. 2"). 
With Diazona violaceum, Pyrosoma gigan- 
teum, Botryllus polycyclus, Salpa maxima, and 
bicaudata, the mantle is without these elegant cells, 
and in the basement-substance are found only 
granules and nuclei, and with Diazona, in addi- 
tion, are pigment-granules, and “rystalline points, or 
calcareous concretions, 
