138 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



2. Skin. This consists of the epidermis and dermis. The 

 epidermis is made up of a single layer of columnar cells (ciliated 

 in the larva), among which are a number of scattered sensory 

 cells, each of which is somewhat rod-like and terminates exter- 

 nally in a stiff tapering process, while internally it is continuous 

 with a nerve-fibre. These cells are most numerous in the anterior 

 part of the body. 



The derm-is presents a firm external layer, beneath which is a 

 much thicker gelatinous stratum again succeeded by a very thin 

 nucleated layer. 



3. Skeleton (Figs. 38 and 39). As in all Chordates the most 

 important part of this is internal, constituting an cndoskeleton, 

 which contrasts strongly with the cuticular exoskeletons of such 

 forms as Crayfish and Mussel. Amphioxus, however, possesses 

 only a feeble endoskeleton, of which the most characteristic part 

 is the notochord. This is an elastic rod which runs from one end 

 of the body to the other, above the gut and below the central 

 nqrvous system. It gives a certain amount of firmness to the 

 body, and serves for the attachment of the lateral muscles. The 

 notochord is of cellular nature, as is readily seen in young speci- 

 mens, but most of the constituent cells become, later on, much 

 vacuolated, so that their outlines are obscured. They are arranged 

 so as to form a succession of thin vertical discs. 



The notochord is surrounded by a firm connective-tissue sheath 

 which is continued dorsally into a tube investing the spinal cord, 

 and laterally into septa running between the myomeres and join- 

 ing the dermis. The notochordal sheath resembles the dermis in 

 structure, the dense layer being in this case internal, while the 

 other layers are continued into the neural sheath and septa. 



Amphioxus possesses other skeletal structures besides those 

 already described i.e., a buccal skeleton, a branchial skeleton, 

 and fin-rays. 



The buccal skeleton consists of a series of short rods jointed 

 together so as to form an incomplete ring stiffening the margin 

 of the mouth, with processes extending into the buccal tentacles. 

 These parts resemble the notochord in minute structure. 



The branchial skeleton supports the pharynx, and will be described 

 in the next section. 



The dorsal Jin is supported by a very large number of minute 

 vertical fin-rays, consisting of little columns of gelatinous material 



