II. 6 



SKELETAL STRUCTURES OF AMPHIOXUS 



29 



probably serve to protect the body during the rapid dives by means of 

 which the creature enters the sand. The habit of swimming with the 

 front end downwards suggests the presence of a gravitational receptor 

 mechanism. The larvae of lampreys swim in a similar way (p. 114). 



scut 



Fig. 7. Section of the skin of amphioxus. 



b.v. blood-vessel; cut. cutis; ep. epidermis; n. nerves; s.cut. sub-cutis. 

 (After Krause.) 



5. Skeletal structures of amphioxus 



Around the notochordal sheath is a further layer of gelatinous 

 material containing fibres. There are no cells within this material 

 but it is secreted by cells around the outside, which retain the epithe- 

 lial arrangement of the mesoderm from which they were derived. 

 This connective tissue continues as a sheath around the nerve-cord 

 and above this into a series of structures known as fin-ray boxes, 

 which support the median ridge. These are more numerous than the 

 segments and each contains a more rigid material referred to as 

 'cartilage'. The relationship of these structures to the fin supports of 

 vertebrates is obscure. Other skeletal rods occur in the cirri around 

 the mouth and in the gill bars. 



6. Skin of amphioxus 



The epidermis differs from that of vertebrates in being very thin, 

 composed of a single layer of cells, ciliated in the young, and with the 

 outer border slightly cuticularized in the adult (Fig. 7). It is not 



