242 



Arthropoda. 



205. 



Fig. 204. 



also valves at the limits of the chambers. The heart, as in other 

 Arthropods, lies in a spacious cavity, the pericardium, which is. 

 bounded above by the dorsal wall of the 

 abdomen, below by a perforate plate of con- 

 nective tissue interlaced with muscle fibres. 

 The heart and a tubular extension of its 

 anterior end, the aorta, are the only vessels, 

 the blood circulates in spaces between the 

 organs in a fairly regular current. After 

 traversing tbe body it enters the peri- 

 cardium, and from this, in consequence of the 

 dilatation of the heart and the opening of the 

 ostia, into the heart itself, whence it is again 

 driven out through the aorta into the sinuses 

 of the body. 



Genital organs. The female, as in 

 other Arthropoda, possesses a pair of ovaries. 

 Each consists of a varying number of tubules; 

 (ovarioles) which usually extend like fingers 

 from the anterior end of the oviduct. Bach 

 ovariole is surrounded by a thin membrane 

 and is immature anteriorly, consisting of 

 small homogeneous cells ; further back there- 

 are larger cells, young ova, lying in the- 

 middle of the tube, and surrounded by 

 smaller cells, which provide them with 

 nutriment and also secrete the shell 

 (chorion), for the fully developed egg. The 

 mature ova occupy the posterior ends of the 

 ovarioles, and pass thence into the oviduct ; 

 when an egg passes into the latter the cor- 

 responding portion of the ovarian tubule- 

 shrinks, and thus the egg next in front is 

 brought nearer to the duct. The two 

 oviducts unite to form an unpaired 

 portion, the vagina,* which opens ventral 

 to the anus, either freely on the surface 

 or into a cloaca, an invagination occurring at the hinder end of 

 the body. There is usually an evagination of the vagina which 

 serves as a receptaculum seminis and one, or a pair of 

 accessory glands which secrete either a sticky fluid to attach the ova 

 to foreign bodies, or the mucus surrounding them {e.g., in Insects 

 which lay their eggs in water) ; sometimes there is also an evagination 



Pig. 204. Portion of the 

 heart of an Insect, dia- 

 grammatic, i constriction 

 between t-wo chambers,' fc 

 Talves, s Tenons ostia. — 

 Orig. 



Pig. 205. Ovariole of 

 an Insect, diagrammatic. 

 ce young o-\nim, ce' mature 

 ovum, s shell, r empty lower 

 extremity of the ovarian 

 tubule (an egg has just 

 escaped). — Orig. 



* In some fe-w Insects {Thysanwra, Ephemera), the vagina is -wanting, and both 

 ducts open direct on the postero- ventral surface of the body (c/., Crustacea). 



