BODY FLUIDS AND CIRCULATION 107 



important factor, varying in significance in different groups, in maintain- 

 ing an efficient circulation suited to the animal's requirements. Both 

 cardiac rate and amplitude are usually subject to regulatory mechanisms, 

 described in more detail in sections to follow. 



Heart rates vary with temperature and Q l0 values range between 2-3 

 in many species. Like certain other activities, there is evidence that heart 

 rates tend to show temperature adaptation (acclimatization) in various 

 species. This is established for the limpet Acmaea Umatula, and is implicit 

 in the adaptation of rate function to latitude shown by many forms. For 

 example, the rate of pulsation of the dorsal blood vessel of Perinereis 

 cultrifera has been compared in specimens at Plymouth (England) and 

 Tamaris (Mediterranean). The rate in northern specimens was the same at 

 14°C as in Mediterranean specimens at 20 C C (respective summer environ- 

 mental temperatures). In a similar manner Crustacea from northern lati- 

 tudes show faster heart rates than more southern forms, when comparison 

 is made with the same or closely allied species at a given intermediate 

 temperature. Differences in rate processes obtain even in a single locality. 

 Thus, limpets (Acmaea limatula) from the low inter-tidal region have 

 faster heart rates than high littoral animals at a given temperature of 

 measurement. Adaptation of rate processes is one aspect of compensation 

 for variable environmental temperature (18, 52, 112, 117, 138). 



Initiation and Regulation of Cardiac Activity. Hearts are charac- 

 terized by rhythmic and continuous contractility, the initiation and main- 

 tenance of which are primary intrinsic functions. The classical object of 

 cardiac research is the vertebrate heart, in which the beat is demonstrably 

 muscular in origin (myogenic). There are certain invertebrate groups in 

 which cardiac contractions are dependent on the nervous system (neuro- 

 genic hearts). Myogenic and neurogenic hearts are distinguished on 

 several grounds. Myogenic hearts are usually inhibited by acetylcholine 

 in low concentrations (< 10 -8 ), while neurogenic hearts are usually 

 accelerated. Search should reveal ganglion cells in the latter type of heart, 

 or in the immediate vicinity. The electrocardiogram of the myogenic heart 

 consists of regular slow potentials, while that of the neurogenic heart 

 shows fast oscillations. Other suggestive features are ease of tetanization, 

 simultaneity or sequence of contraction throughout the organ, and degree 

 of autonomous activity versus central nervous control. A differential 

 effect of ether on the activity of the two types of hearts is explained by the 

 greater sensitivity which the c.n.s. shows to this anaesthetic compared 

 with heart muscle (26, 94, 108). 



Myogenic Hearts 



The hearts of vertebrates and molluscs are myogenic, and the beat 

 originates in the cardiac musculature. Ontogenetically, contractions appear 

 in the vertebrate heart before it is innervated, and automatic contractions 

 sometimes continue in fragments of adult heart tissue which lack ganglion 

 cells (6). 



