156 INVERTEBRATE PHYSIOLOGY 



servable reactions to stimuli and that these shght changes actually differ- 

 entiate muscles and even the muscle fibers functionally. Hence a welter of 

 phenomena is encountered which makes analysis much more difficult. Of 

 course, this very same differentiation is in itself a highly interesting prob- 

 lem with many connotations, especially for such fields as transmission by 

 synapses and integration. But it is no longer possible to think of a given 

 special mechanism as typical for a group or phylum ; instead, the whole 

 array of variations has to be considered. We have as yet no good indica- 

 tion of how widely varied the mechanisms are in the arthropods nor how 

 much of this variability is present in other phyla. It is obvious that a much 

 more intensive study of all kinds of neuromuscular transmissions will have 

 to be made before it will be possible to compare intelligently the different 

 phyla with each other. It can only be hoped that the use of impaled muscle 

 fibers will provide a way of gathering data at a much increased rate. How- 

 ever, the mechanical aspects of contractions should not be neglected for a 

 comprehensive picture. There is here a field wide open for investigation, 

 in which much more research is necessary than is at present being per- 

 formed. Let us hope that this research will be done ; for it will be a re- 

 warding task, not only for its own sake but also for a better understanding 

 of problems of wider scope. 



Since the foregoing paragraphs were written, a number of papers about 

 Coelenterates have appeared. These can be only summarily discussed. Hor- 

 ridge (1955 a,b,c ; 1956) continued his investigations in different medusae, 

 using neuromuscular preparations and histological investigations, but 

 without the study of the electrical phenomena. He found that in hydro- 

 medusae Geryonia proboscidalis and Aequoria forskalca the swimming 

 movements are governed by a circular through-conducting nervous sys- 

 tem. In Geryonia this system is not influenced by the radial system, which 

 is responsible for the movement of the manubrium by tonic contractions 

 of its radial musculature towards a place stimulated on the bell. But in 

 Aequoria contraction of the radial musculature has an inhibiting effect 

 on the through-conducting system as in a number of other hydromedusae. 

 In the scyphomedusa Rkizostoma pulmo repetitive stimulation causes a 

 shortening of the refractory period in a part of the muscle fibers of the 

 circular muscle of the bell. In the scyphomedusae Cyanea and Cassiopea 

 the compensatory contractions made to keep the animals in a vertical po- 

 sition during swimming were studied. It was found that tonic contractions 

 are caused by a second nervous net, the diffuse net, which acts locally and 

 delays the relaxation time. In order to explain the all-or-none contraction 

 of swimming due to the through-conducting nerve ring and the tonic con- 

 tractions of the compensatory contractions, Horridge postulates the pres- 

 ence of double motor innervation of the muscle fibers. Ross (1955) has 



