178 
MOLLUSCOIDA. 
TUNICATA. 
AsCIDIIDiE. 
According to O. Hertwig^s researches^ the mantle of the 
Tunicata is composed of three layers : — 1. The outer or tunica 
externa j known to contain cellulose, is originally a homoge- 
neous cuticular formation (not a permanent membrane of the 
egg), at first without cells, and remaining so in Doliolum and 
Appendiculariuy but in the Ascidia becoming afterwards real 
conjunctive or cellular tissue by immigration of cells from 
within. 2. The intermediate layer, consisting of epithelial cells, 
arranged in a simple continuous stratum, to which in Phallusia 
semifluid cellular tissue is added. The intercellular sub- 
stance of this layer is either homogeneous, with star-shaped 
and vesicular cells, as in Phallusia ; or fibrous, with spindle- 
shaped cells, and others smaller and filled with pigment. This 
layer corresponds to the epidermis of other animals ; and the 
“ polygonal epithelium in the mantle of the Ascidia of other 
authors belongs to it. 3. The internal skin of the body, tunica 
interna. At both openings of the body the external layer is 
somewhat continued inwards over the internal. The same cel- 
lulose tissue is also found in the muscular bag and in the intes- 
tine of Cynthia mytiUgera. The muscular system consists of 
crossing, smooth muscular fibres — viz. two strata of longitu- 
dinal fibres, and an intermediate stratum of transverse fibres. 
The blood-vessels situated in the interstices of these fibres have 
no proper walls, and take their origin from the internal layer, 
like the stolones at the base of the mantle. Jen. Z. Nat. vii. 
(1871), pp. 46-73, pis. 7-9. 
li. Hert wig's anatomical researches have been directed chiefly 
to the perithoracic cavity, endostyle and abdominal canal, in- 
testine and liver of the Ascidia. The endostyle is provided n 
the middle of its base with cilia, and is regarded as constituting, 
with the abdominal canal, a sensitive organ, although no nerves 
have as yet been detected in it. The numerous cylindrical tubes 
surrounding the intestine, which have been regarded as biliary 
vessels or liver by Prof. Krohn, are blood-vessels, according to the 
author; and the honey-yellow organ described by Krohn is the 
renal organ or kidney. The true liver is a glandular appendage 
of the stomach, and has been already correctly described and re- 
cognized by Savigny in Cynthia, but afterwards mentioned only 
by Milne-Edwards. Ibid. pp. 74-102, pis. 7-9. 
A. Kowalewsky's paper cited in Zool. Rec. viii. p. 176, treats 
chiefly of the development of Phallusia. mammillat a (Cuv.). 
C. Kupffer observes that in Molgula affinis, simplex (A. H.), 
and a new species, there is no larval stage, but the animal. 
