THE CIRCULATION OF BODY FLUIDS 107 



and Satacke (19 12) measured the pressure of the blood in the 

 abdominal aorta of the lobster by means of a water mano- 

 meter, using hirudin as anticoagulant, and found that with 

 the heart beating at a rate of 51 per minute the average 

 pressure-reading was only about 8 mm. of mercury. 



The most extensive studies on the circulatory system of 

 any invertebrate are the investigations of Carlson on the heart 

 of the king-crab, Limulus. In this arachnid' the heart retains 

 the original segmental character. It attains to a length of about 

 six inches in a full-sized animal, and thus provides excellent 

 experimental material. There are eight pairs of ostia. The 

 major arteries are located towards the anterior end of the 

 heart. Histologically it is a syncytium of striated fibres. On 



Fig. 25. — Innervation of heart of Palinurus (Carlson). 



its dorsal aspect there is an elongated median nerve-ganglion, 

 which can be easily detached. There are also a pair of laterally- 

 disposed nerves connected with the abdominal gangHa and 

 with the cerebrothoracic ganglion of the central nervous 

 system. Electrical stimulation of the extrinsic cardiac nerves 

 from the abdominal ganglia leads to acceleration of the normal 

 rhythm ; the same result may be produced by stimulating 

 the ventral nerve-cord or abdominal ganglia themselves after 

 transection of the cord behind the brain. Stimulation of the 

 brain or its nerve-connections with the heart leads to diastolic 

 arrest. There is thus a double augmentor-inhibitor mechanism 

 by which the activity of the heart is subjected to control by 

 the central nervous system, as in vertebrates. The same has 

 been shown to be the case in Maia (Bottazzi, Polimanti) and 

 Palinurus (Carlson) among decapod Crustacea. 



