96 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



consists of plasma, in which are suspended numerous colourless cor- 

 puscles, nucleated, and capable of performing amoeboid movements. 

 On the dorsal side of the thorax is a good-sized pericardial sinus, 

 in which the heart is suspended by fibrous cords. This organ is 

 a flattish, thick-walled sac, into which open three chief pairs of 

 valvular apertures (ostia), dorsally, laterally, and ventrally respec- 

 tively. From the heart seven delicate arteries* proceed, five in 

 front and two behind. From the front a small impaired ophthalmic 

 artery runs forwards and divides to supply the eyes, antennules, 

 and cerebral ganglia, while on each side of this a much larger 

 antennary artery arises which supplies the antenna, giving off 

 branches to the stomach during its course. A hepatic artery takes 

 origin a little behind each antennary artery, and supplies the 

 mid-gut and digestive gland. The posterior end of the heart 

 dilates into a kind of bulb which is continued backwards into a 

 dorsal abdominal artery running in the middle plane above the 

 intestine, to which it gives oif numerous branches. Near the 

 point of origin of this vessel a sternal artery leaves the heart, 

 gives a branch to the reproductive organs, and, running down- 

 wards on one side or other of the intestine, pierces the nerve- 

 cord, and divides into an anterior ventral thoracic artery and a 

 posterior ventral abdominal artery, both of which run below the 

 nerve-cord. 



The arteries branch repeatedly, and some of the branches enter 

 into capillary plexuses, as, for example, upon the cerebral ganglia. 

 The finest ramifications end in minute spaces (lacunce) found in 

 all parts of the body and which communicate with larger spaces 

 (sinuses) that are placed around the various organs. The most 

 important of these is the sternal sinus, running along the ventral 

 surface, and with which the others are directly or indirectly 

 connected. There are external channels in the gills which com- 

 municate on the one hand with the sternal sinus, and on the 

 other hand with internal gill-channels opening into branchio-cardiac 

 canals which run up the sides of the thorax into the pericardial 

 sinus. 



The heart is chiefly made up of interlacing muscular cords, 

 which in their turn are aggregates of elongated muscle-cells, the 

 inner sides of which are metamorphosed into transversely striated 

 contractile substance. 



* An artery is a blood-vessel carrying blood away from the heart. 



