208 
MOLLUSCOIDA. 
homologous with the visual organs of the Arthropodaf the single eye of 
Pyrosoma is furnished with a lens, and may be likened to those of some 
Mollusca. The author is satisfied that the Tunicata are not Mollusca ; he 
enumerates briery the chief differences between both, and the resem- 
blances of the former to the types of the Vermes and Vertehrata^ and gives 
the preference to O. Schmidt’s view that the Tunicata form a special 
class, Protovertehrata. 
A tubular gland belonging to the intestinal tract, found in most 
species of Tunicata^ is described by T. Chandelon [supra, p. 206] : its 
functions are affirmed to be digestive. 
Lacaze-Duthiers has observed in young specimens of Molgula, that 
the mantle is formed by exudation from the ectoderm of the larva, 
after this has fixed itself, and that the vessels in it come from ray-like 
prolongations (villosit^s) of the ectoderm penetrating into the mantle. 
C. R. Ixxx. pp. 600-604 ; R. Z. (3) iii. p. xi. 
K. B. Reichert gives an anatomical description of the larva of 
Botryllus violaceus observed at Trieste ; he states that in the normal 
situation of the swimming larva, the branchial sac is above, and the 
heart, intestine, and genital organs are beneath, on account of their 
weight, and he therefore purposely avoids the terms “ dorsal ” and 
“ ventral.^’ Three front appendages, probably for fixing the animal, and 
eight others, called girdle appendages, are described. Concerning the 
skin, the author distinguishes first, an extremely thin homogeneous stra- 
tum, called by him testa, and a thick cellular stratum or wall, which lies 
in a part of the body immediately under the testa, but in the anterior 
part forms the fioor of the branchial sac, this latter being situated be- 
tween the cellular wall and the testa, and therefore not quite internal ; 
in other parts of the body, there are some small cavities between the 
testa and wall, which are sometimes filled by foreign bodies, and have 
been incorrectly described as testa. The tail consists of the following 
parts : — 1, innermost, the axial string (Achsenstrang), homogeneous, not 
cellular, without cover, not entering into the main part of the body ; 2, 
a stratum of spindle-shaped contractile cells which cause the movements 
of the tail by their contraction, whereas the elastic axial string tends to 
re-establish the straight position ; 3, the testa, which envelops the tail 
all round, but is elevated in the median plane above and below into a 
vertical fin, and this fin shows oblique linear thickenings which have the 
appearance of the fin-rays in a fish. The cellular stratum is composed of 
eight longitudinal bands, each being a row of hexagonal cells. The 
author thinks that the axial string is a product of the secretion of the 
cellular stratum, and therefore calls it an “ internal cuticular formation.” 
That which has been described by Kupfer as a spinal nerve is, according 
to Reichert, probably an artificial slit or chink between the longitudinal 
bands of the cellular stratum and the hexagonal cells of these^ bands. 
He is therefore of opinion that the tail of the Ascidian larva exhibits a 
striking analogy to, but by no means a homology with, that of a fish or 
tadpole. Abh. Ak. Berl. 1875, pp. 131-196, 6 pis. • 
, A. Giard has discussed the question of the affinity of the Ascidian 
larvas to the Vertehrata, applying the principle of abbreviation of the 
