no COMPARATIVE PHYSIOLOGY 



the common ancestor of Limulus and the Crustacea in a form 

 which, like Peripatus, possessed only smooth muscle, and 

 therefore, presumably, a heart whose rhythm was myogenic. 

 In support of the neurogenic interpretation of the crustacean 

 heart beat Hoffmann (19 12) has emphasised certain peculiari- 

 ties which are shared by the electrical response of the crustacean 

 and arachnid heart. In both cases each major variation 

 corresponding to a single mechanical contraction displays 

 small superimposed oscillatory deflections. It is, however, 

 to be noted that the exposed heart of the crustacean beating 

 in situ often shows twitching movements superimposed upon 

 the regular beats ; and the same is true of the isolated heart 

 beating in sea water or other suitable media. In connexion 

 with the supposed neurogenic origin of the heart-rhythm in 

 Crustacea, it is worthy of note that the frequency of the beats 

 has a temperature coefficient which is fairly high, as may be 

 seen from the following data taken from Brailsford Robertson *s 

 paper on Ceriodaphnia : 



Ql0ll72I° 1*92 



Qioi3723° 1*90 



Q1015725'' 2' 20 



Qioi9729° 2- 18 



The Circulatory System in Molluscs. — Among the Cepha- 

 lopod molluscs the circulatory system reaches the highest 

 degree of specialisation met with among the invertebrate 

 phyla. The anatomical relations are briefly as follows. 

 Venous blood is collected by the caval vein which bifurcates 

 to form two branches supplying the gills. These afferent 

 branchial vessels dilate at the base of the gills into rhythmically 

 contractile branchial hearts, which drive the colourless venous 

 blood through the gill-capillaries. Oxygenated blue blood is 

 collected by the efferent vessels directly to the auricles of the 

 systemic heart, the ventricle (which may be divided) pumping 

 the arterial blood into the aortae. This would seem to be a 

 much more efficient device for supplying oxygenated blood to 

 the tissues at high pressure than the arrangement which exists 

 in fishes, where the force of the heart's heat has to overcome 

 the resistance of the gill-capillaries as well as that of the body 



