206 ZOOLOGY. 



sessile condition ; t indicates tlie body-wall, consisting of 

 epithelial cells. 



We will now, from the facts afEorded 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 Awgust. After the larva fastens itself 

 by the three processes to some object, the cliorda 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 quadrangTi- 

 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 2:)ericardium. 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 AmpMoxus, Saqitta, Phoronis, Echinus, etc. The first 



