134 ESSENTIALS OF ZOOLOGY 



Sections will show that the ectoderm is a single layer of 

 columnar cells which secretes a cuticle externally and is based 

 on a thin layer of connective tissue or dermis internally. 



INTERNAL MORPHOLOGY. The main features of structure 

 can be made out by dissection, but it is better to examine 

 prepared specimens of young and sections of the adult. 



The connective tissue which forms a thin dermis under the 

 ectoderm is continued into septa between the myotomes, and 

 these internally are continued in the sheaths which surround 

 the nerve cord and the notochord, and with fasciae which 

 separate the muscles from the cavities below. The connective 

 tissue also supports the median fins ; it is disposed in the form 

 of a series of compartments filled with a gelatinous fluid, by 

 means of which the fins are kept distended. The connective 

 tissue, in short, is spread throughout the body, and it is con- 

 tinued from the one region and purpose to the next. 



The myotomes are made up of horizontal muscle fibres, 

 and these are striated. Each fibre is enclosed in a delicate 

 connective-tissue investment, and the fibres pass from septum 

 to septum in succession all along the body. They thus form a 

 series of muscles on each side, extending from above the mouth 

 to the end of the tail. It will be observed also that the 

 myotomes of one side alternate in position with those of the 

 other. The body-cavity region is long and the tail region short, 

 and the presence of the myotomes so far forward allows of 

 lateral movements throughout the whole length of the body, 

 movements employed in swimming and in burrowing, and an 

 Amphioxus may penetrate the sand either forwards or back- 

 wards. The myotomes are retained in a simple, unchanged 

 state in Amphioxus. In the Craniata they are more or less 

 fused and modified to form special muscles related to special 

 actions, but all Craniates pass through a stage similar to that 

 of the adult Amphioxus. In Amphioxus, however, the floor 

 of the atrial cavity is provided with a sheet of transverse 

 muscles, the function of which is to compress the atrial cavity 

 and thus help in expelling the water. 



The notochord, the essential characteristic feature of the 

 vertebrate, is present also in Amphioxus. It extends from 

 end to end of the body. It lies just above the central line 



