526 COMPARATIVE ANATOMY chap. 



In the Oy^jhophtJuclmidtE, and especially in the genus Gihlocellum, 2 long Malpighian 

 vessels are found which enter the sac-like expanded rectum (cloaca). Each vessel 

 begins with a blind terminal tube, which breaks up into a plexus of fine tubules, 

 uniting again into a single vessel entering the rectum. Malpighian vessels have been 

 found in many Acarina. They are generally in the form of 2 long, occasionally coiled 

 tubes, entering the hind-gut. Sometimes the 2 tubes unite in a common duct which 

 enters the hind-gut, and they thus assume the form of the letter Y (Atax). In other 

 Acarina the excretory organ is an unpaired tube lying on the mid-gut. In 

 Sydrodoma it emerges close behind the anus, but separate from it. In other cases 

 numerous Malpighian vessels are said to enter the hind-gut near the anus (Argas). 

 Here and there a rectal sac like that of the Araneidce is found, and into this enter 

 both the gut and the Malpighian vessels (Gamasidce, Fig. 369, C, and JSalarachnidce). 

 The arrangement of the long Malpighian vessels in the larva and the first nymph 

 stage of the Oamasidce is interesting. They here (Fig. 361, p. 514) reach far forward 

 and form a loop at each leg which may reach into its third or fourth joint. The 

 blind ends of the two vessels reach far into the first pair of legs. In some Acaridce 

 and in the LinguaUilidce no Malpighian vessels have as yet been found. Our know- 

 ledge of the Malpighian vessels of the Arachnoidea in general is exceedingly scanty. 



VI. The Blood-vaseular System. 



Among the Arachnoidea this system shows very various stages of 

 development. It is most highly developed in the Scorpionidm and next 

 in the Araneidce. The blood nowhere flows entirely in blood vessels 

 separated from the body cavity, but rather for a larger or smaller 

 Dortion of its course enters blood sinuses and lacunae, which represent 

 the coelome. In the Arachnoidea also distinct relations between the 

 blood -vascular system and the respiratory organs can be established. 

 "Where the respiratory organs are very strictly localised, as in the book- 

 leaf tracheae of the Scorpionidce and Araneidce, the vascular system with 

 walls of its own is most developed ; where the respiratory organs are 

 dispersed over the whole body, as they are in the Antennata, and also 

 where there are no special respiratory organs, the peripheral portion 

 of the vascular system is reduced, as in the Antennata, and even its 

 central organ, the heart, may disappear. 



The central organ, the heart (Figs. 370, 371), shows, like that 

 of the Crusktcea, various degrees of concentration, from the extended 

 many-chambered dorsal vessel provided with numerous pairs of ostia 

 (Scorpionidce), to the short, one-chambered cardial sac with one pair of 

 ostia (Acaridce). This progressive concentration is evidently closely 

 connected with the progressive concentration of the whole body. 



That the heart lies in a pericardium has only been with certainty 

 observed in a few cases. Muscles and strands of connective tissue, 

 which are attached on the one side to the heart or the pericardium 

 (the latter appears to be the case in the Araneidce), and on the other to 

 the integument, seem to occur pretty generally. 



After the heart itself the most constant portion of the vascular 

 system is a median anterior vessel-like prolongation of the heart, 

 running on the dorsal side to the brain ; this may be called the aorta 



