XIV. INTRODUCTION. 



of the structure of a typical Simple Ascidian, dealing chiefly 

 with those points which are of importance in classification 

 and diagnosis. The figures on Plate A. illustrate the follow- 

 ing description of an ASCIDIA : 



The ordinary species of Ascidia, such as are found in both 

 European and Australian seas, in shallow waters round the 

 coast, are irregularly ovate bodies, several inches in length, 

 attached by one (the posterior) end to a stone or other 

 foreign body (PI. A., fig. 1). The opposite (anterior) end has 

 an opening, Br. (the "branchial aperture" or mouth), sur- 

 rounded by 8 lobes, and leading (fig. 4) by a funnel-shaped 

 passage (the "branchial siphon") into a large cavity (the 

 "branchial sac " or pharynx), which occupies a great part of 

 the interior of the body (fig. 2). The wall of the branchial 

 sac is perforated by numerous slits (the "stigmata," formed 

 by the breaking up or modification of gill slits) by means of 

 which the water taken in at the mouth passes outwards into 

 a space (the " peribranchial cavity' or " atrium "), which 

 nearly surrounds the branchial sac and communicates with the 

 exterior on the " dorsal ' ' surface by an opening (the " atrial 

 aperture") surrounded by 6 lobes (figs. 1, 2, and 4). The 

 positions of the branchial and atrial apertures serve to 

 distinguish the anterior end and dorsal surface. 



These branchial and atrial apertures are the only 

 openings on the surface of the body (fig. 1), which 

 is covered all over by a firm and sometimes thick layer, 

 the "test," which is of the nature of a cuticular secretion 

 organised by the immigration of numerous cells (fig. 5), 

 some, at least, of which are mesodermal. The ectoderm, the 

 outer epithelium or continuous layer of cells of the body, 

 lines the test and separates it from the muscular and con- 

 nective-tissue body wall, known as the "mantle." Out- 

 growths of the mantle covered by ectoderm (fig. 5) may form 

 branching blood "vessels" in the test (PI. B, fig. 9). These and 



