6o 



ASCIDIANS 



has formed the beginning of the muscular body-wall, the con- 

 nective tissue around the organs, and the blood; the endostyle has 

 developed as a thick-walled groove along the ventral edge of the 

 pharynx, which has become the branchial sac ; and the peri- 

 cardial sac and its invagination the heart have formed in the 

 mesoblast between the endostyle and stomach. The " epicardiac 

 tubes " grow out from the posterior end of the endostyle to join the 

 pericardium. They play an important part 'in the formation of 



Fig. 26. — Metamorphosis of an Ascidian. A, free - swimming tailed larva ; B, the 

 metamorphosis — larva attached ; C, tail and nervous system of larva degenerating ; 

 D, further degeneration and metamorphosis of larva into E, the young fixed Ascidian. 

 at. Atrial invagination ; ch, notochord ; hy, hypoblast cells ; i, intestine ; m, mouth ; 

 mes, mesenteron ; n.c, neural canal ; n.v, neural vesicle with sense-organs. (Modified 

 from Kowalevsky and others.) 



buds in the colonial Tunicata. The heart acquires a connexion 

 with blastocoelic blood -spaces at its two ends. The heart 

 and pericardium show the same relations in Tunicata as in 

 Enteropneusta, but it is very doubtful whether these organs are 

 genetically related to the Vertebrate heart. 



The unpaired optic organ in the cerebral vesicle when 

 fully formed has a retina, pigment layer, lens and cornea ; while 

 the ventral median organ is a large, spherical, partially- 

 pigmented otolith attached by delicate hair-like processes to 

 the summit of a hollow "crista acustica " (Fig. 26, A). After 

 a few hours, or at most a day or so, the larva attaches itself 



