io6 COMPARATIVE PHYSIOLOGY 



quarter of an hour later. This consists of a jellying of the 

 plasma, but according to Tait (191 8) it is accompanied by 

 a further cytolysis of corpuscles, which he designates thigmo- 

 cytes. Loeb states that lobster tissues yield specific coaguHns 

 for the blood of the same species. According to Tait all the 

 corpuscles of crustacean blood are actively phagocytic. The 

 subject has attracted a large number of investigators since 

 attention was focussed upon it by the pioneer labours of 

 Fredericq ; but much still remains to be done, especially 

 from the standpoint of a more modern appreciation of colloidal 

 behaviour. 



The Ostiate Heart o£ Limulus and Crustacea. — Investi- 

 gation into the circulatory system of Arthropods has been 

 chiefly focussed upon the task of elucidating the origin and 

 conduction of the cardiac rhythm. The essential characteristic 

 of the arthropod circulation is the ostiate heart. Generally 

 speaking the heart is composed of striated muscle fibres, and 

 lies in a capacious sinus into which, in those forms which lead 

 an aquatic life, there discharge the lacunar spaces of the gills ; 

 in the tracheates the circulatory system is poorly developed, 

 though an ostiate heart is present. 



There is no division of the heart into auricular and ventri- 

 cular portions. It is filled by way of valve-like apertures, 

 the ostia, which communicate directly with the pericardial 

 sinus during diastole, but remained closed during the systolic 

 phase. From the heart proceed the larger arteries. In 

 Crustacea, at least in Decapods (Baumann), there are valves 

 at the cardiac end of the larger arteries, so that the pressure 

 in the arterial system does not fall to zero in diastole. A well- 

 developed arterial system is found in Limulus, scorpions and 

 decapod Crustacea. In those forms in which the arterial 

 system is well developed the heart is a powerful organ, and 

 beats with a frequency comparable to that of the cephalopod 

 heart. 



Nevertheless, from such data as we have at our disposal, 

 it would seem that the peripheral resistance is very low, doubt- 

 less on account of the fact that the arteries discharge into 

 lacunar spaces and not into a network of capillaries. Brucke 



