174 



ANIMALS 



that besides the epithelial layers which cover the exterior of the 

 animal and line the internal cavities, there are also other tissues 

 like the muscles which lie between the ectoderm and entoderm. 

 These other tissues constitute a third layer, which is called 



Fig. 89. — Structure of the body-wall of hydra. A, Part of a cross section 

 of the column; B, the region between'^ectoderm and entoderm, more highly 

 magnified to show the longitudinal and circular muscle fibres; C, diagram of 

 an ectoderm cell with a muscle-fibre process. Ect, ectoderm; En, entoderm; 

 /, muscle fibre; N, nematocysts; s, supporting layer between ectoderm and 

 entoderm. 



mesoderm (in the embryonic stage of development). This layer 

 is entirely wanting in hydra and is not well developed in any 

 of the Ccelenterates. 



409. The muscles of the Annelids are composed of fibres, 

 and each fibre consists of a bundle of muscle fibrils. Each 



Fig. 90. — A branched muscle fibre from the wall of a blood-vessel (Nereis). 



fibre has a nucleus and represents an elongated cell. The fibres 

 taper to a point at each end and are arranged parallel in 

 masses called muscles. In the worm the principal muscles are 

 arranged in two sets. Just beneath the epidermis there is a 

 thin layer which runs circularly around the body, and beneath 



