358 



TUNICATA. 



According to SEBLIGER, the lens and the meniscus develop within a single 

 cell derived from the wall of the sensory vesicle. KOWALEVSKY, on the 

 contrary, believes that they arise from three cells, and this agrees with 

 LEUCKART'S statement (No. 37) that three biconvex lenses are found within 

 the pigmented eye-cup. 



Fia. 168. Anterior region of the body in the free-swimming larva of Plwllusia. 

 mammillata (after KOWALEVSKY). A, lateral aspect; B, dorsal aspect, au, eye; 

 b, blood-sinus between the gill-clefts , ch, chorda ; cl, cloacal aperture ; d, alimentary 

 canal ; ed, intestine ; es, endostyle ; f, ciliated pit ; h, adhesive papillae ; i, mouth 

 (branchial aperture) ; k', k", first and second gill-slits ; m, caudal muscles ; ms, 

 mesenchyme-cells ; of, otolith ; r, trunk-section of the medullary tube ; s, caudal 

 section of the medullary tube ; sb, sensory vesicle. 



The so-called auditory organ (Fig. 168, ot) consists of a pear-shaped 

 or conical cell (otolith-cell) projecting into the interior of the sensory 

 vesicle, the narrowed end of which is inserted between the cells of 



