254 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
6. THE LATERAL LINE SYSTEM OF THE STURGEON. 
This structure is not mentioned by systematic writers, or at least by none that I 
have been able to consult. In specimeus 9 inches in length the lateral line is trace- 
able as a distinct canal which perforates the posterior lateral plates just below the 
longitudinal ridge which extends along their obter faces. It is traceable from the 
exterior as far forward as the vertical cutting across the dorsal and anal fins, and 
opens by way of pores to the exterior between the edges of the last two or three 
lateral plates. Behind the last lateral plates the lateral line suddenly changes its 
course and follows the general inclination of the upper or longest lobe of the tail, 
traversing a narrow armored strip of the lateral caudal integument just below the 
lateral caudal armature, which consists of a pavement of small rhombic plates, with- 
out strong carinse or points. In the young animal, the lateral line developed along 
the sides of the upper lobe of the tail is a simple tubular canal traversing the integu- 
ment, and at short intervals it opens to the exterior by way of pores. 
In the adults it is not so easy to trace the lateral line forward, but on the sides 
of the upper lobe of the tail it may be observed to open by a series of pores in close 
proximity to the site of the lateral line in the young, but the pores are no longer 
linear in their arrangement, indicating that the canal has branched and gives off short 
lateral branches as adolescence is attained. This description applies to A. sturio. 
It is unfortunate that the failure to obtain a complete set of the stages of develop- 
ment will not enable me to give a fuller account of the ontogeny of this structure in A. 
sturio. Fortunately, in a memoir by N. Zograff (Studies from the Zoological Labora- 
tory of the Museum of the University of Moscow, in Russian, t. Lii, pt. 3, Moscow, 
1887, p. 44), the lateral line of the head and trunk of A. ruthemis is figured at a stage 
which shows that there is a supra- and infraorbital, occipital, lateral, rostral, and supra- 
opercular system of canals with punctiform end organs. This arrangement of the 
lateral sensory nerve-hills is probably very easily made out in quite young stages of 
the sturgeon, before the epidermal lateral canal closes and sinks down into the deeper 
layer (corium) of the skin. A careful dissection of the lateral line region of young 
individuals of A. sturio 8 to 9 inches in length shows that the lateral branch of the 
vagus nerve is present along the middle line of the side as a very conspicuous cord, 
just beneath the lateral row of scales and slightly embedded in the lateral muscular 
mass, invested by a connective tissue sheath. Careful inspection also shows that the 
lateral scutes along the .sides as far forward as the opercle are perforated by a canal, 
which is supplied, as described by N. Zograff, in A. ruthenus, with small twigs from 
the lateral branch of the vagus nerve. These small twigs terminate in groups of hair- 
cells — nerve hills — forming part of the ectodermal lining of the system of lateral canals. 
The structure of the system of canals on the head is of the same character, but the end- 
organs and canals are not as numerous and complex as in Amia and many Teleosts. 
Another sensory apparatus of some complexity are two pairs of barbels in front of 
the mouth. These appear in A. sturio to be innervated by a large sensory branch of 
the fifth nerve, which is very easily traced upon removing the integument on either 
side of the middle line, on the under side of the snout. The barbels themselves are 
considerably longer in proportion in the young sturgeon of 10 inches to 2 feet than in 
the full-grown A, sturio measuring 7 to 10 feet in length, while the transverse incisures 
