ii6 ZOOLOGY FOR MEDICAL STUDENTS chap. 



enteron or alimentary canal and coelome or body -cavity, and (2) by 

 the fact that the coelenteron communicates with the exterior by a single 

 opening — the primitive mouth (protostoma). 



The body-wall of the coelenterate consists of the two primary layers 

 of cells, ectoderm and endoderm, separated by the jelly-like or membranous 

 mesogloea. This latter is primarily a secretion formed by the activity 

 of the two layers of cells bounding it but where it becomes bulky, as in 

 the umbrella of the Medusa or the ground-substance of the colony of 

 the Alcyonarian or the body-wall of the Anemone, it usually becomes 

 colonized by cells of the primary cell-layers (most usually of the ectoderm) 

 which have taken on an amoeboid character and wandered into it. 

 These cells, which collectively constitute the mesenchyme, are of import- 

 ance in connexion with the subsequent evolution of the Metazoa, for 

 in many of the more highly evolved groups of animals the mesenchyme 

 forms a large proportion of the entire bulk of the body and becomes 

 specialized to form important tissues. 



The two primary cell-layers are especially interesting in the coelen- 

 terate from the fact that they display to us incipient stages in the evolution 

 of the muscular and nervous systems. 



The myo-epithelial cell like that of Hydra is apparently the starting- 

 point for the evolution of the muscle fibre as it occurs in the more complex 

 animals. In such a comparatively actively moving coelenterate as a 

 medusa the myo-epithelial cells on the under surface of the umbrella 

 frequently have their cell-body reduced to the nucleus, with a small 

 quantity of cytoplasm round it, lying beside the greatly enlarged con- 

 tractile cross-piece. The myo-epithelial cell has in such a case actually 

 become a muscle flbre. 



The coelenterate nervous system consists of the scattered sensory 

 cells, the ganglion-cells which have sunk inwards and lost their sensory 

 hair, and the plexus or network by which sensory and ganglion-cells 

 are linked together. As has already been indicated the Medusae show 

 an advance in the evolution of their nervous system in the aggregation 

 of sensory cells into groups so as to form definite sense-organs. Another 

 feature of the Medusae is that they show a tendency for ganglion-cells 

 also to be concentrated together in the neighbourhood of the margin 

 of the umbrella to form nerve centres. In the Acalephae such a nerve 

 centre is developed in the neighbourhood of the attached base of each 

 sensory tentacle. From these centres emanate the stimuli that bring 

 about the contraction of the muscle fibres and hence the rhythmic 

 pulsations of the umbrella. If the tentacles with a small adjacent portion 

 of the umbrella are carefully excised from the living Aurelia its pulsations 



