140 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The first nerve is dorsolateral and runs beneath 

 the apodeme of anterior oblique muscle 6 on its 

 way to the muscles of the sixth abdominal seg- 

 ment. The second nerve serves the uropod exopo- 

 dite, the third the endopodite, and the fourth, 

 which arises posterodorsally, runs near the anal 

 canal and enters the telson. These nerves are ap- 

 parently mixed. The exopodite nerve (fig. 72, B) 

 is easily observed. After giving off branches to 

 the exopodite muscles, this nerve undergoes exten- 

 sive subdivision to form a kind of sensory "cauda 

 equina" in the distal regions of the exopodite. 



III. CIRCULATORY SYSTEM 



The typical blood circulatory system of Crus- 

 tacea Decapoda is a closed arterial-open venous 

 system. It consists of a heart and arteries by 

 which blood is transported to arterial capillary 

 beds throughout the body. Venous blood returns 

 to the heart through conjoint blood sinuses in the 

 appendages and body into which the capillaries 

 empty. In general, the gills are served by the 

 venous system. Although the circulatory systems 

 of all decapods have marked similarities, the 

 arteries display greater differences in detail than 

 the nervous system or the muscle system. Fur- 

 thermore, the circulatory elements are variable 

 within the individual animal. The blood vascular 

 system will be treated in two parts, the heart and 

 pericardium and the blood vessels. 



A. Heart and Pericardium 



Figures 7!>, SO 



The pericardium is a thin, contractile mem- 

 Inane surrounding the heart. The membrane is 

 penetrated by passageways by which venous blood 

 may enter the pericardial chamber. Slender 

 muscle bundles inserted on the surface of the peri- 

 cardium aid in its rhythmic contractions. 



The heart of Pena&us setiferus is a many-sided 

 structure lying within the muscular pericardium 

 in the posterodorsal region of the carapace. Three 

 pairs of valvular afferent ostia may be seen on 

 the surface of the heart, two pairs dorsally and 

 one pair placed laterally. By means of the valves 

 in these pores, pericardial blood enters the heart 

 during diastole and is prevented from escaping 

 in the systolic beat. Structurally, the heart is 

 more than a tubular expansion, and is, instead, 

 comprised of a system of connected sinusoids. 



Microscopical sections indicate that the outer and 

 intersinusoidal walls are made up of muscle fibers 

 interspersed with connective tissue of a type strik- 

 ingly reminiscent of the wavy elastic fibers in the 

 tunica media of a vertebrate artery. 



Although the details of cardiac innervation have 

 not been worked out in the present study of Pen- 

 aeus, the work of Alexandrowicz (1932) shows 

 that considerable uniformity in the heart nerves 

 of decapod crustaceans exists. Alexandrowicz has 

 found that the decapod heart is well supplied with 

 nerves. Restricted to the internal heart structures 

 is a local system of heart neurons interspersed 

 among the muscles of the sinusoidal septa. Alex- 

 androwicz considers the local system inherently 

 automatic. 



The heart neurons are connected to the central 

 nervous system by a pair of dorsal cardiac nerves 

 which arise from the subesophageal ganglion. 

 The dorsal nerves Alexandrowicz believes to be 

 regulatory and he subdivides them functionally 

 into two groups of inhibitory and one of accelera- 

 tor fibers. Distinct from the heart neurons and 

 dorsal nerves is a third system serving the muscles 

 of the pericardium and heart valves. The peri- 

 cardial nerves are ventral and the ostial valve 

 nerves are situated dorsally, and the two are inter- 

 connected with one another. Inexplicably, the 

 pericardium-ostial valve system is not tied to 

 either the heart neurons or the dorsal cardiac 

 nerves. Consequently, Alexandrowicz states, the 

 pericardium has its own rhythm. 



B. Blood Vessels of the Body 



LATERAL ANTERIOR ARTERY 

 Figures 79, 80 



The narrow, apical end of the heart points ros- 

 trally. From this point in the midline a very small 

 vessel extends a short distance anteriorly (fig. 80). 

 This little artery may be the vestige of the ophthal- 

 mic artery (Huxley 1906), or the median aorta 

 (Baumann 1919) found in Astacus and other crus- 

 taceans. In the opinion of Professor Mayrat 

 (letter, May 3, 1957), ophthalmic (or median 

 aortic) vascular function has apparently been 

 taken over by a pair of lateral anterior arteries 

 (figs. 79, 80) arising from the anterior end of the 

 heart, laterad of the midline. 



As the lateral anterior arteries run forward, 

 they give off a number of branches, the first of 

 which leaves just anterior to the heart. The 



