ANATOMY AND DEVELOPMENT. 1 83 



A schematic representation of a dissection of a typical 

 Ascidian after Professor W. A. Herdman, whose reports on 

 the Ascidians collected during the voyage of H. M. S. 

 Challenger have done so much to advance our knowledge 

 of the group, is given in Fig. 94. The greater part of the 

 thick cartilaginoid test (also called tunic, outer mantle, or 

 cellulose mantle), t, is supposed to be removed from the 

 right side, and its cut edge can be traced all the way round. 

 Below the test comes the inner or muscular mantle, w, 

 which is the true body-wall, to which the external tunic is 

 secondarily superadded.^ The muscular mantle is limited 

 externally (below the test) by the epidermis, and beneath 

 the latter are the interlacing muscle-fibres which compose 

 the bulk of the mantle. 



Beneath the mantle is an extensive cavity surrounding to 

 a large extent the viscera. This is the peribrancJiial or 

 atrial cavity which communicates with the exterior by the 

 atrial or cloacal aperture, at.s. 



The mouth, or.s, leads into the pharynx or branchial sac, 

 ph, which is of surprising dimensions, and stretches nearly 

 to the posterior end of the body. The walls of the bran- 

 chial sac are perforated by innumerable gill-openings, the 

 so-called stigmata, arranged in successive transverse rows, 

 through which the water which enters at the mouth passes 

 out of the sac into the atrial cavity. 



Dorsal Lamina, Endostyle, and Peripharyngeal Band. 



On cutting through its right wall we open into the 

 cavity of the branchial sac along the dorsal side of which 

 a fold is seen projecting freely into the cavity, the so-called 

 dorsal lamina corresponding to the dorsal groove in the 

 pharynx of Amphioxus, while along its ventral side is a 



