396 ZOOLOGY. 
sessile condition; ¢ indicates the body-wall, consisting of 
epithelial cells. 
We will now, from the facts afforded us by Kowalevsky, 
trace the changes from the larval, free-swimming state to 
the sessile adult Ascidia, which may be observed on the 
New England coast in August. After the larva fastens itself 
by the three processes to some object, the chorda dorsalis 
breaks and bends, the cells forming the sheath surrounding 
the broken axial cord. The muscular fibres degenerate into 
round cells and fill the space between the chorda and the 
tegument, the jelly-like substance forming a series of wrin- 
kles. With the contraction and disappearance of the tail be- 
gins that of the nerve-vesicle, and soon no cavity is left. The 
three processes disappear ; the pharynx becomes quadrangu- 
lar ; and the stomach and intestine are developed, being 
bent under the intestine. A mass of cells arises on the an- 
terior end beneath the digestive tract, from which originate 
the heart and pericardium. In a more advanced stage, two 
gill-holes appear in the pharynx, and subsequently two more 
slits, and about this time the ovary and testis appear at the 
bottom, beyond the bend of the alimentary canal. The free 
cells in the body-cavity are transformed into blood-cells, and 
indeed the greater part of those which composed the nervous 
system of the larva are transformed into blood-corpuscles. 
Of the embryonal nervous system there remains a very small 
ganglion, no new one being formed. The adult Ascidian 
form meanwhile has been attained, and the very small indi- 
viduals differ for the most part only in size from those which 
are full-sized and mature. 
It will be seen that some highly important features, recall- 
ing vertebrate characteristics, have occurred at different pe- 
riods in the life of the embryo Ascidian. Kowalevsky remarks 
that “‘ the first indication of the germ, the direct passage of 
the segmentation cells into the cells of the embryo, the for- 
mation of the segmentation-cavity, the conversion of this 
cavity into the body-cavity, and the formation of the diges- 
tive cavity through invagination—these are all occurrences 
which are common to many animals, and have been observed 
in Amphioxus, Sagitta, Phoronis, Echinus, etc. The first 
