80 INVERTEBRATE MORPHOLOGY. 



lio-rnuscular, muscular, glandular, and nerve cells are gener- 

 ally to be found in it. The epithelio-muscular cells (Fig. 40, 

 A) are the most numerous and consist of columnar cells, one 



extremity of which bears a cilium and 

 helps to form the outer surface of the 

 body, while the other is prolonged into 

 a somewhat spindle-shaped process of 

 highly contractile muscular substance. 

 The muscle-cells (Fig. 40, ) are modi- 

 fications of these, having lost their 



FIG. 40.- = Epithelio- connect ion with the surface of the body, 

 muscular cell of Ccelen- 



terate B = muscular the cell-protoplasm and nucleus form- 

 cell ; C = sensory cell. ing a small elevation on the muscle- 

 fibre. The muscle-fibres rest upon the 



outer surface of the mesoglcea, and in the ectoderm are, as a 

 rule, arranged longitudinally, so that by their contraction 

 they cause a shortening or retraction of the polyp. The nerve- 

 cells are of two kinds: (1) sensory cells (Fig. 40, 6*), which are 

 slender cells whose free end bears a single cilium, while 

 the inner end is produced into one or more slender nerve-proc- 

 esses which are supposed to place these cells in connection 

 with (2) the ganglion-cells. These are stellate cells lying in 

 the deeper layers of the ectoderm, just external to the muscle- 

 cells, and sending off delicate processes in various directions 

 so as to form a plexus of nerve-fibres ramifying through the 

 ectoderm. 



The mesogloaa is thin and more or less fibrous in structure, 

 and rarely contains cells. The eudoderm-cells are large and 

 are of the epithelio-muscular variety, the muscle-fibres hav- 

 ing a circular arrangement producing by their contraction a 

 diminution of the diameter of the polyp. The protoplasmic 

 portion of each cell is furnished with a single flagellum and 

 is digestive in function, taking food-particles into its sub- 

 stance and there digesting them. In addition to these, gland- 

 ular cells also occur in the eudoderm, especially in the region 

 of the hypostome. 



The medusa forms have the shape of a bell (Fig. 41) 

 which may be either shallow or deep, the mouth of the 

 bell in all cases being partially closed by a fold of tissue 



