196 



PHYSIOLOGY OF THE INVERTEBRA7A. 



The Arachnida (pulmonary) have a circulatory apparatus, 

 which is to a certain extent well developed. The heart 

 (Fig. 42), situated dorsally, has the form of an elongated 

 vessel, and gives rise to various arte- 

 ries. The blood having traversed the 

 organs, passes to the lungs, and from 

 thence reaches the heart, following a 

 course similar to that observed in the 

 Crustacea. In those Arachnida which 

 breathe only by tracheae {e.g., the 

 mites), the circulatory apparatus is 

 rudimentary ; for there appears to be 

 merely a simple dorsal vessel without 

 arteries or veins ; and it may be re- 

 marked, that in some species the heart 

 or dorsal vessel appears to be entirely 

 wanting. 



In some of the lower Crustacea 

 the heart is entirely absent. Tor in- 

 stance, in the Cojpepoda there is no 

 heart ; while in the Ostracoda there is 

 either no heart (Cypris and Cytherc), 

 or it is only in a rudimentary form. According to 

 Claus, the heart in Cypridina, Halocryptis, and Conchcecia 

 consists only of a short saccular organ with one anterior and 

 two lateral appendages. The Cirripedia have no heart or 

 other circulatory apparatus — that is, as far as is known in 

 the present state of biological science. 



Dr. G. 0. Sars* has recently investigated the circulatory 

 apparatus in Cyclestheria Mslopi (see Pig. 11), one of the 

 Phyllopoda. The heart of this Phyllopod is located in the 

 dorsal part of the body, and is easily observed in living 

 specimens through the transparent shell. It has the 

 form of an elongated tube traversing no less than four 

 segments of the body, viz., the maxillary segment and the 

 * Christiania Videnskahs-Sehlcahs Forhandlinger, 1887. 



Fig. 42. 

 Heart of a Spider/ 



a = abdomen, i = lateral 

 pulmonary vessels, c = ante- 

 rior aorta. d = transverse 

 branches, e — genital arteries. 



