STRUCTURE OF THE VERTEBRATES 301 



End organs are more complex. The simplest type is the tactile 

 corpuscle, the nerve ending at a tactile cell surrounded by a 

 connective tissue cup. In other types the nerve may end in a 

 minute plexus resembling a glomerulus and surrounded by a 

 sheath. The largest corpuscles are those described by Pacini 

 (Pacinian corpuscles) in which the nerve core is surrounded by 

 concentric layers of connective tissue. These are easily demon- 

 strated in sections of the mesentery, or the pancreas of the cat. 

 They are large enough to be located without a lens. 



A. Lateral Line Organs 



In addition to the isolated cutaneous sense organs, the cyclo- 

 stomes, fish, and gill-bearing amphibia have epithelial sense 

 organs arranged in definite lines upon the head and body. The 

 distribution of these lines varies greatly, although there is a 

 basic arrangement of the primitive lines which is added to or 

 modified in many ways. These lines radiate from the region 

 of the ear. Two are facial, one above and one below the eye, 

 converging toward the snout; a third is mandibular; and the 

 fourth, which gives the name to the group of organs, lies along 

 the axis of the body. This last follows generally the horizontal 

 line separating the epaxial from the hypaxial regions of the 

 muscle segments. The lateral line is parallelled by the lateral 

 branch of the vagus nerve. 



During development the organs appear as conical patches of 

 modified epithelium, the base of the cone resting upon the der- 

 mal tissues. The central cells are sensory, each having a flagel- 

 lum-like bristle projecting from the epidermis, and are sur- 

 rounded by the supporting cells of the cone. The fibrils from 

 the lateral nerve pass through the supporting cells and ramify 

 around the sensory group. 



In the cyclostomes and amphibia each cone sinks into a pit 

 which is not directly connected with other depressions. In the 

 fish these pits are modified to form connecting grooves or canals. 

 In one group of the cartilaginous fishes the canals remain open 

 in the adult; in the others the canals sink deeper and the lateral 

 ridges close over to form canals which open to the outside through 

 pores located above the sensory patches. The position of the 



