COELENTERATA. 



101 



FIG. 81. Ectoderm cells of Hydra with con- 

 tractile processes (rm). From Chun, after 

 Kleinenberg. 



contractile processes (Fig. 81) arranged along the long axis of the 

 body; and cnidoblasts, which form the thread-cells, or nematocysts, 

 and carry sensory processes, the cnidocils or triggers, projecting on the 

 surface. The thread-cells (Fig. 

 82) are small capsules consisting 

 of a highly refractile cuticular 

 material, and containing some 

 fluid and a spirally - coiled 

 thread. Under certain mechan- 

 ical conditions, e.g., under slight 

 pressure produced by contact 

 with a foreign body, these 

 capsules suddenly protrude the 

 thread, which either fastens on to the causes of the disturbance, or 

 pierces it, carrying into it a part of the fluid contents of the capsule. 

 In many parts of the body, and especially on the tentacles, which 

 serve for the capture of prey, these microscopic weapons are present 

 in great numbers, and are often 

 grouped in a peculiar arrangement 

 to form batteries of thread-cells. 



The endoderm cells are principally 

 concerned with the processes of 

 digestion and secretion. They often 

 bear cilia for the movement of the 

 contents of the gastrovascular space, 

 and their deeper ends are sometimes 

 prolonged into contractile processes 

 which are transversely arranged. 

 The endoderm of the tentacles is 

 sometimes solid, and modified for 

 a skeletal function by the develop- 

 ment of vacuoles and cuticular struc- 

 tures into a form resembling the 

 vegetable parenchyma of plants, or 

 the tissue of the vertebrate noto- 

 chord (Fig. 83). The food is 

 digested in the Protozoan manner, 

 being surrounded by protoplasmic processes of the endoderm. The 

 digestion is therefore intracellular, and the indigestible remains are 

 cast out into the enteron and ejected through the mouth by the 

 help of the cilia of the endoderm cells. 



FIG. 82. a, b Nematocysts and cnido- 

 blasts of Cordilophora, with the cnidocil 

 of the cell (cnidoblast) ; c adhesive cells 

 of a ctenophore (from Lang). 



