CONNECTIVE AND SKELETAL TISSUES. 91 



The muscular fibres seem to be unstriated, but Bateson de- 

 tected signs of a faint transverse striation in Dolichoglossus 

 koivalevskii. 



In addition to the muscles above described, muscular fibres 

 are present in the walls of the branchial pouches and in the 

 anterior wall of the collar coelom. 



The connective and skeletal tissues and basement membrane. 

 The Balanoglossida present the remarkable feature of having 

 no connective tissues in the ordinary sense of the word, excepting 

 the delicate reticulate tissue found in the coelomic cavities and 

 extending amongst the muscles. This connective tissue is, as 

 already mentioned, a product of the walls of the embryonic 

 coelomic sacs and appears to be of the nature of mesenchyme. 

 On the other hand all the epithelia of the body possess a kind 

 of internal cuticle, that is to say they secrete on their inner- 

 surfaces a continuous structureless membrane devoid of nuclei 

 and fibrous structures. This structureless layer is called the 

 basement membrane. It is formed not only on the internal 

 side of the ectoderm cells but also on the inner side of the endo- 

 derm and of the walls of the coelomic sacs, even in cases in which 

 these walls have become entirely converted into connective 

 tissue and muscles. It follows from this that not only must 

 there be a layer of basement membrane interposed between all 

 the organs of the body, but also that this interposed membrane 

 must, theoretically at any rate, be in all cases a double membrane, 

 one of its laminae being derived from one and the other from 

 the other of the two organs which are in contact. As a matter 

 of fact this doubleness of the basement membrane is not as a rule 

 discernible, the two laminae of which it is theoretically composed 

 having completely fused. But a separation persists in places 

 and the cavities thus formed constitute the blood vessels (see 

 below). 



The skeletal tissues are entirely derived from this membrane. 

 The proboscis skeleton has already been described. Here it is 

 only necessary to call attention to the fact that one part of 

 this skeleton the chondroid tissue differs from the rest of 

 the basement membrane of the body in containing strings and 

 nests of nuclei which have migrated into it from the adjacent 

 coelomic walls. 



The branchial or pharyngeal skeleton consists of a special 



