652 



CIRCULATION. 



Fig. 325. 



Insects. 



any other parts (Jig. 325 **, , v). A net- 

 work of vessels is also distributed over the 

 surface of the imperfectly formed wings. As 

 the metamorphosis from the larval to the 

 perfect state advances, and shortly after the 

 insect leaves the water to assume the aerial 

 condition, the circulation of the blood be- 

 comes gradually confined to a more and 

 more circumscribed space. The loops extend- 

 ing into the wings, limbs, caudal processes, 

 and antennae, become shorter; when the meta- 

 morphosis is complete, they become entirely 

 closed, and in general this change is followed 

 by the disappearance of the inferior lateral or 

 returning currents also. These remarkable 

 changes in the circulatory organs at once indi- 

 cate an interesting relation of their condition to 

 the changes in the mode of life of the insect. 

 In the aquatic state, the caudal and lateral 

 laminae, antennae, and wings may be considered 

 as serving the purposes of gills, for the blood 

 is carried to them, and exposed upon their 

 surfaces to the action of the water. The larvae 

 of the neuropterous insects generally feed 

 largely, but their life during the perfect condi- 

 tion, when the circulation has ceased, is of 

 short duration, and they either take very little 

 food, or live in absolute abstinence. It has 

 been also shewn that the dorsal vessel consists of 

 different compartments, between each of which 

 a valvular apparatus (fig. 325*, .r) prevents the 

 passage of the blood in a retrograde direction. 

 There are lateral openings in the neighbourhood 

 of the valves, by which it would appear that 



the blood is admitted into the dorsal vessel from 

 cross branches (Jig. 325**, y) passing directly 

 from the lateral streams. It may be mentioned 

 that the larger returning streams of blood, situ- 

 ated on the lower side of the body, are said by 

 Carus and Wagner, we cannot judge with what 

 reason, not to be inclosed within vascular pa- 

 rietes, but to run loose in the texture of the 

 insect. A complete circulation is not, how- 

 ever, confined to the larvae of insects, having 

 been discovered by Carus and others in some 

 of the perfect insects. Carus saw it in the 

 wings of the Semblis developed for flight. The 

 circulation has also been seen by Carus in the 

 larvae of Water-beetles, Hydrophilus, and Dy- 

 tiscus, and by Ehrenberg and Hemprich in the 

 Mantis, so that the circulation has now been 

 discovered in insects belonging to four orders, 

 viz. Coleoptera, Diptera, Orthoptera, and Neu- 

 roptera. 



Crustacea. In the Stomapoda, Isopoda, and 

 Branchiopoda, or in the Squill, Oniscus, and 

 Monoculus or Daphnia, the circulation is ge- 

 nerally described as being of the same simple 

 kind as that just stated to occur in the larva of 

 insects, with this exception, that the blood is 

 carried to gills for the purpose of undergoing 

 a respiratory change. In most of them the 

 venous blood which is sent to the gills comes 

 directly from the systemic veins. From the 

 description given by Gruithuisen of the circu- 

 lation in the Daphnia,* it would appear, if his 

 observations are correct, that the venous blood 

 is sent to the heart before going to the gills, 

 a distribution very dissimilar from that which 

 exists in the rest of the articula'.ed animals. In 

 this animal, Gruithuisen also describes an au- 

 ricle and ventricle in the heart. 



The investigations of Messrs. Audouin and 

 Milne Edwards have pointed out very clearly 

 the structure of the circulatory organs and the 

 course of the blood in the larger Decapodous 

 and some other Crustacea. The aortic heart 

 (Jig. 326, H), consisting of a single ventricular 

 cavity, and situated below the posterior margin 

 of the thoracic shield, gives off six systemic 

 arteries (A, fl), which convey the arterial blood 

 to the various organs of the body and to the 

 liver (/*). The venous blood, returning thence 

 in the systemic veins (v, v), is collected on the 

 lower surface of the body into sinuses ( V, V), 

 from which the branchial arteries (B) take their 

 origin ; the branchial veins (6) return the blood 

 which has passed through the gills to the heart. 

 Arachnida. In those of the Arachnida in 

 which the respiratory organ consists of tracheae 

 like that of insects, the circulation has been 

 supposed to be much the same as in these 

 latter animals. The dorsal vessel, however, 

 approaches to the form of a heart posteriorly, 

 being there more dilated at one part than in 

 the rest of its course, and considerable lateral 

 vessels are known to be given off from it upon 

 either side. In others of the spiders, in which 

 the respiratory organ consists of pulmonary ca- 

 vities admitting air, it is conjectured that the 

 blood is distributed on the surface of the plates 



* Nova Acta Nat. Cur. xiv. p. 404. 



