IX 



OTOCYST, LATERAL LINE 



345 



endolymphatic duet of the Dogfish is replaced by a bhndly ending endo- 

 lymphatic sac. 



In the lower part of the otocyst are contained large otoliths — masses 

 of small crystals of calcium carbonate lightly bound together by organic 

 material and readily disintegrating into their constituent particles. 



As will have been gathered, the otocyst is enclosed within the 

 cartilage of the auditory capsule. The narrow space between otocyst 

 wall and cartilage is occupied by lymph (perilymph) and is traversed by 

 occasional fine strands of connective tissue. 



Lateral-line Sense-organs. — Whereas the three pairs of sense- 

 organs described so far are found throughout the whole series of verte- 



MTA VDix^ 



Fig. 144. 



Diagram showing the arrangement and nerve supply of the lateral-line sense-organs in a shark 

 (after Ewart). The sensory canals are dotted ; the ampullae are shown as small circles ; the nerves 

 are shown in black, a, Ampullae ; olf, olfactory organ ; y.c.I, spiracle ; VII, facial ; VIIo^, super- 

 ficial ophthalmic branch of facial ; VII6, buccal branch of facial ; Vllhm, hyomandibular branch of 

 facial ,' IX, glosso-pharyngeal ; X, vagus. 



brateSj those now to be described — the sense-organs belonging to the 

 lateral-line system — are peculiar to the lower, water-inhabiting, verte- 

 brates. These sense-organs are small in size, very numerous and are 

 dotted over the surface of the body and head. Each organ consists of a 

 small clump of sensory cells, known as a neuromast, the cells being of 

 the usual columnar shape with a projecting sensory hair at their outer 

 end. Primitively these sense-organs are on the surface of the body but 

 as a rule they become sunk beneath the surface and are found in the 

 adult at the dilated inner ends (ampullae) of deep pits, or along the bottom 

 of a groove which may become covered in to form a tube (sensory canal) 

 running along parallel to the surface and retaining at intervals a com- 

 munication with the exterior. The cavity of the tube or pit is kept 



