376 TUNICATA. 



It is interesting to compare the transformations . undergone by the 

 Ascidian larva after attachment with that of the Cirripedia and the 

 Pedicellina larvae, in which to some extent analogous conditions 

 prevail. 



A degeneration similar to that undergone by the organs of the 

 caudal region also takes place in the larval nervous system. The 

 anterior sensory vesicle breaks down, its elements become spherical 

 and lose their connections. For some time after the sensory organs 

 have disappeared a mass of pigment remaining from them may be 

 observed in the body-cavity of the young Ascidian. The degenerated 

 elements later reach the circulatory system, where they probably 

 completely disintegrate (p. 364). A similar disintegration is suffered 

 by the tissue on the ventral side of the trunk-ganglion which consists 

 of large ganglionic cells. The central nervous system of the young 

 Ascidian, according to VAN BENEDEN and JULIN (No. 9), consists of 

 those elements which surround the central canal in the region of the 

 trunk-ganglion, and are continued anteriorly on to the left swelling 

 of the sensory vesicle. The elements derived from the cephalic vesicle 

 thus yield the definitive ganglion, while the elements of the trunk- 

 ganglion produce aganglionic cell-strand (cordonganglionnaire visceral] 

 discovered by VAN BENEDEN and JULIN, which, first running back- 

 ward in the dorsal median line, becomes applied to the dorsal wall of 

 the pharynx but then diverges to the left, runs along the left side of 

 the oesophagus and ends between the two hepatic diverticula. After 

 the gelatinous cover has been perforated by the apertures of ingestion 

 and egestion, the admission of water and of food into the alimentary 

 canal becomes possible. 



In the further development of the branchial network, the chief im- 

 portance attaches to the appearance of new slits, each of which, as M 

 rule, arises through the fusion of a shallow diverticulum growing out 

 from the entodermal wall of the pharynx with the lining of the atrial 

 cavity. At the point of fusion, the slit is first visible as a very small 

 aperture. In this way, according to KOWALEVSKY, in Phallusia 

 mammillata, after the first gill-slit has formed, a second appears 

 behind it (Fig. 168, k") this being apparently of the same size as 

 the first. Later, according to KROHN (No. 33), two new slits appeal- 

 between these two and behind the last slit (the second in order of 

 formation) one more. In this way five primary gill-slits form in a 

 longitudinal row. Each of the trabeculae between every two gill-slits 

 contains a blood-sinus (branchial vessel, Fig. 168, b}. Above and be- 

 low this primary row of slits, other rows are said to form later, the 



