196 



PHYSIOLOGY OF THE INVERTEBRA7 A. 



>6 



Fig. 42. 

 Heart of a Spider. 



The Arachnida (pulmonary) liave 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 (''.//., 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 



a = abdomen, b = lateral the heart is entirely absent. For in- 

 pulmonary vessels. ^ = ante- ^^^^^q i^ the Compoda there is no 

 rior aorta. d = transverse 



branches, e = genital arteries, heart; wliile in the Ostrcicoda there is 



either no heart {Cr/iwis and Cytlicrc), 

 or it is only in a rudimentary form. According to 

 Claus, the heart in Cypridina, Halocr]i})tis^ and Conclioicia 

 consists only of a short saccular organ with one anterior and 

 two lateral appendages. The Cirrijx'dm 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 Cychstlteria hislojn (see Fig. 11), one of the 

 Pliyllopoda. 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 

 * Christionia V'ulenslcahs-Selslcahs ForhanclKnger, 1887. 



