PHYSIOLOGY OF ASCIDIA ATRA LESUEUR Zo" 
under similar conditions. The frequency with which Salpa 
pulsates is several hundred times as great as those with which 
Ascidia and Ecteinascidia perform their spontaneous contractions. 
These facts as well as the apparent lack of function of the 
rhythmic movements have led me to suggest that perhaps the 
rhythm is the degenerate remains of a once vigorous activity. 
The ascidians are generally supposed to have originated from the 
free swimming appendicularians. These possess no tentacles, 
and most probably the earliest ascidians did not possess them 
either. It is, therefore, entirely intelligible that the rhythmic 
discharge of the water from the branchial sac of these ancestral 
ascidians was of considerable value as a cleansing process. More- 
over, the salpas are derived from the early ascidians. With 
their specialization for a pelagic existence, the rhythmic move- 
ments were developed into a mechanism for respiration, feeding 
and locomotion. The stem line of ascidians, however, soon 
developed tentacles, and the rhythmic discharge of the bran- 
chial sae contents, therefore, decreased in importance. The 
frequency with which it occurred probably also decreased. On 
the basis of this hypothesis, there exist at present two divergent 
lines of development of the spontaneous movements. One of 
these constitutes the salpas, whose frequency of contraction is 
several hundred times greater than that of the ascidians, which 
constitute the other line. 
V. SUMMARY 
1. The blue-black color of Ascidia is due to the presence of 
spherical pigment granules, which are the metamorphosed re- 
mains of the green blood cells. Before becoming imbedded in 
the test, the green cells turn blue, and may be found as such in 
the blood stream. 
2. Ascidia is capable of regenerating its test. The process of 
regeneration and the normal sloughing of the test show that there 
is a continuous secretion of material on to the inner face of the 
test. 
3. A species of amphipod lives commensally in the branchial 
sac of Ascidia. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 25, NO. 1 
