WHITE SHRIMP FROM THE GULF OF MEXICO 



149 



antennal muscles and enters the antenna proper 

 with the antennal nerves. Here it divides to 

 supply the antennal scale and endopodite seg- 

 ments. 



VENTRAL APPENDAGE BLOOD SUPPLY 



The ventrally located appendages of the head 

 and gnathothorax all appear to receive blood from 

 branches of the ventral thoracic artery. Those 

 of the abdomen, as has already been mentioned, 

 receive fluid from the segmental branches of the 

 dorsal abdominal artery. 



D. Venous System 



Arterial circulation consists of a closed afferent 

 system subdivided into capillary beds in muscles 

 and other organs. Venous return to the heart ap- 

 pears to be carried out in an open system, through 

 sinuses in the appendages which empty into larg- 

 er sinuses in the hemocoel. In general, the ap- 

 pendicular sinusoids are found on either one mar- 

 gin or the other of the appendage, rather than in 

 the middle. The sinuses of the body which drain 

 the venous blood from the appendages are located 

 in particular places in the body. The sinus into 

 which the eyestalks empty occupies the anterodor- 

 sal regions of the protocephalon. This sinus runs 

 into a larger one anterior to the gastric mill into 

 which blood from the other protocephalon append- 

 ages drain. The largest thoracic sinus lies along 

 the sternum. It receives blood from the dorsal 

 sinuses and the gnathothoracic appendages and 

 supplies blood to the respiratory organs. From 

 the sternal sinus the venous blood passes dorso- 

 laterally to the pericardium. 



In the abdomen, the sinus system is relatively 

 simple. The dorsal regions are drained by a sinus 

 or set of sinuses between the dorsal and ventral 

 abdominal arteries. These appear to run directly 

 rostrad into the heart. On the ventral surface 

 segmentally arranged, conjoint sinuses pass venous 

 blood rostrally to the sternal sinus of the thorax 

 and thence to the heart. The sternal sinuses of the 

 abdomen extend some distance dorsolaterally m 

 conjunction with the sinuses of the pleonic ap- 

 pendages. 



E. Respiratory System 



The subject of gill formulae and arrangement 

 in Penaeus and other crustaceans has been dis- 



cussed previously in the general section on the 

 gnathothorax. The details of the respiratory sys- 

 tem will be taken up at this point, since the organs 

 concerned with gas exchange are intimately as- 

 sociated with the circulation of body fluids. 



The gills of Penaeus are of the dendrobranchi- 

 ate type (Caiman 1909) . This type of gill is com- 

 prised of a primary axis or rachis from which 

 pairs of secondary structures bearing gill filaments 

 arise at right angles. The secondary structures 

 and gill filaments thus appear to be laminar units. 

 Proceeding distally, each succeeding layer of sec- 

 ondary structures and filaments nests within the 

 preceding layer. 



For the details of gill circulation we are in- 

 debted to Prof. Jerome E. Stein, Texas Agricul- 

 ture and Median ical College, Galveston, Tex. He 

 finds that blood enters the primary rachis of the 

 gill by means of a primary afferent blood vessel 

 (fig. 78, A, B) which is separated from the pri- 

 mary efferent blood vessel by a longitudinal sep- 

 tum. Primary afferent blood is directed into the 

 secondary afferent blood vessels (fig. 78, G) by a 

 secondary septum. Blood now passes out into the 

 secondary structures to the gill filaments, into 

 which it runs due to the arrangement of a tertiary 

 septum dividing the tertiary afferent and efferent 

 blood vessels in each filament. Blood thus flows 

 around the tip of the filament and returns to the 

 sternal sinus through the efferent vessels of the 

 gill. The gill filament cuticle and epidermis ap- 

 pear to be very thin to allow for gas exchange. 



An important accessory organ of respiration is 

 the branchiostegite. The inner cuticle adjacent to 

 the gills is thin and the underlying epidermal 

 layers are heavily vascularized. Specimens pre- 

 served in Zenker's fluid display with great clarity 

 the branchiostegal vessels (fig. 77) and fine 

 nerves. The vessels appear to be of two types. 

 One set represents the subdivisions of a large 

 vessel seen at the dorsal margin of the branchio- 

 stegite a short distance caudad of the hepatic 

 spine. The capillaries of the vessel approach the 

 periphery of the branchiostegite at right angles 

 to its margin. A second set of vessels run parallel 

 to the margin of the branchiostegite (fig. 77), and 

 are reminiscent of growth rings, since the larger 

 shrimps in general have more rows of these vessels 

 than do the younger animals. Whether the mar- 

 ginal vessels reflect the age of the white shrimp in 

 terms of the number of molts is not known. 



