SKELETON. 



15 



are without cilia except on the cirri, in the mouth and in the 

 atrial cavity, and their outer surface is covered by a porous 

 cuticle. Immediately beneath the ectoderm is a layer of fibril- 

 lated tissue called the cutis. Beneath this comes the subcu- 

 taneous tissue which consists of a gelatinous matrix containing 

 sinuous fibres. The tissue within this has a similar form and 

 extends between the myotomes, as the inter-muscular septa, 

 to become continuous with the sheath of the notochord. In 

 fact all the connective tissues of the body may be said to form 

 a continuous framework which supports the organs and is on the 

 whole of very similar structure tliroughout. In some parts 

 it is firmer than in others and in some places it contains fibres, 

 but it never presents a modification of a cartilaginous or osseous 

 nature and never, except at the ventral ends of the primary 

 pharyngeal bars, contains cells other than the epithelial cells 

 which bound the spaces contained within it. These spaces are 

 in some cases vascular and in others coelomic, but it is not possible 

 in every case to be certain as to which of these two organs they 

 belong. This absence of what we may call mesenchymatous 

 elements from the connective and supporting tissues is one of 

 the most remarkable peculiarities of Amphioxus. 



As skeletal tissue we may rank the notochord, the supporting 

 tissue of the buccal ring and the axial tissue of the buccal cirri, 

 possibly also the axial tissue of the pharyngeal bars. The 

 notochord is made up of a number of discs placed verti- 

 cally, and transversely to the long axis, and consisting of 

 gelatinous tissue. It is surrounded by a tough sheath of 

 connective tissue, which is continuous with the rest of the 

 connective tissue framework of the body. Nuclei are present 

 on the dorsal and ventral sides in the neighbourhood of two 

 spacer, the so-caUed dorsal and ventral Ipnph canals of the 

 notochord. 



The edges of the mouth contain a ring of skeletal tissue the 

 buccal ring, resembling the notochord in structure. It consists 

 of about twelve pi'eces on each side, and each piece gives attach- 

 ment to a rod of the same substance, which occupies the axis 

 and forms the support of one of the oral cirri. 



The tissue of this buccal skeleton consists of a number of gelatinous 

 discs surrounded by a fibrous sheath. By some observers it is claimed 

 as cartilage, each disc being a cell and the smrounding membrane the 



