538 



Vertebrata. 



divisible into a roundisli trunk and a flattened tailj which is 

 folded on to the ventral side. The wall of the capacious pharynx is 

 perforated on each side by a ciliated aperture, the gill-slit, which 

 opens on to the surface ; the rest of the alimentary canal is short, 

 the anus is ventral. The notochord only occurs in the tail, 

 where, however, it is well developed. The central nervous 

 system is represented by a cord swollen into a bulb (the brain) 

 dorsal to the pharynx, and provided with smaller swellings on the 

 rest of its course ; it is continued down the tail on the left side 

 of the notochord, so that the tail is really laterally compressed 

 and has undergone a rotation of 90°. The simple heart lies below 



the alimentary canal. An otocyst is present, 

 but eyes are wanting. 



The simple Ascidians (i.e., those which 

 do not form colonies, genus Ascidia, etc.) 

 are apparently of quite a different structure. 

 They are barrel-shaped, round, or of some 

 other form, and, for the most part, gela- 

 tinous ; they are fixed by one end or by one 

 side. At the free end two openings are 

 present; one of these, the mouth, leads 

 into a very roomy pharynx or branchial 

 sac, the walls of which are perforated by 

 numerous ciliated clefts ; these do not 

 lead directly on to the surface, but into 

 a large peribranchial cavity sur- 

 rounding the branchial sac, and communi- 

 cating with the exterior by the second 

 opening, the cloacal aperture. The 

 branchial sac is fused along one side with 

 the outer wall of the peribranchial cavity, 

 and along this line there is a longitudinal 

 furrow, the ventral furrow or 

 endostyle, with large mucus-secreting 

 and ciliated cells ; along the opposite side 

 runs a dorsal lamina, often con- 

 siderably folded, and connected anteriorly 

 with the endostyle by a ciliated band on 

 each side (the peripharyngeal bands). The 

 pharynx leads below into a rather short 

 intestine, which bends upon itself and 

 opens into the peribranchial cavity. The 

 heart lies below the alimentary canal ; the current of blood is remark- 

 able in that it flows alternately in opposite directions through it ; there 

 is a fairly well-developed system of vessels, the branchial sac being 

 especially well provided ; the blood corpuscles are all amoeboid and 



Pig. 426. Diagrammatic 

 longitudinal section of an 

 Asoidian; the section is not 

 quite median, but somewhat 

 lateral, a anus, cl cloacal aper- 

 ture, g branchial sac, g' aper- 

 tures in its wall, m mouth, n 

 nerve ganglion, p peribranchial 

 cavity, t gut, B ventral, R 

 dorsal surface. — Orig. 



