626 IN VERTEBE AT A CHAP. 



of this groove meet each other, and in this way the atrial cavity 

 of the adult is formed. The larval test becomes detached from the 

 ectoderm covering most of the body, which it hereafter loosely 

 enswathes, and the mouth becomes open. The test, however, remains 

 attached to the body near the posterior end of the endostyle. Here 

 a process is formed which projects into the test, and from which 

 fresh test cells are budded off during life. 



The pericardium arises during free-swimming life. The mode 

 of its development has been satisfactorily made out by Kiihn (1893), 

 who has summarized and corrected the accounts of earlier observers. 

 In the free-swimming larva a thickening of the ventral wall of the 

 pharynx can be observed (Fig. 456, A). This thickening is at first 

 a mere doubling of the cells forming the wall of the pharynx in this 

 region. At the close of larval life it becomes a round mass of cells 

 distinct from the pharyngeal wall. In this mass a cavity appears 

 which opens into the pharyngeal cavity (Fig. 456, B). This cavity is, 

 however, soon cut off from the pharynx, and forms the pericardia! 

 vesicle. Then its dorsal wall is bent inwards as a median fold. The 

 space between the limbs of this fold is the cavity of the heart (Fig. 

 456, E), and this space communicates with the general body -cavity, 

 which is a blood-space, or haeniocoele, formed by the separation of 

 ectoderm and endoderm, and which is a development of the slit-like 

 blastocoele iu the embryo. After metamorphosis the pericardial 

 vesicle enlarges and becomes thin-walled, and from its inner w r alls, 

 which constitute the wall of the heart, muscular fibrils are developed, 

 and then the peristaltic action of the heart begins. 



From the pharyngeal wall, in front of and to the sides of the 

 spot from which the pericardium originated, two hollow outgrowths 

 arise and grow backwards. These are the epicardial tubes or 

 epicardia. They were first described in Ciona by Newstead (1894), 

 whose observations were extended and corrected by Damas (1899). 

 According to Damas the left epicardium is larger than the right. 

 Both grow rapidly and form thin-walled vesicles (Fig. 456, F), which 

 constitute the perivisceral cavity of the adult Ciona, the primary 

 body-cavity being reduced to a series of blood-sinuses. The con- 

 joined inner walls of the epicardia form a kind of visceral peritoneum, 

 enwrapping heart, pericardium, and intestine. From the left epi- 

 cardium a plate of cells extends into the process of the body, from 

 which cells are budded into the test. By this plate the cavity of 

 this process is divided into two channels which serve as afferent and 

 efferent blood-streams. The epicardia in compound Ascidians are 

 closely connected with the process of budding ; their meaning will 

 be considered later. 



When we last considered the pharynx it was provided with two 

 gill-slits on each side, which communicated with the exterior by a 

 lateral ectodermal invagination, the atrial opening. We also described 

 how these two lateral atria become merged in a common median 

 atrial cavity. According to Willey, these primary slits, or proto- 



