330 ARACHNOIDEA AND PAL&OSTRACA. 



then follow one or two pairs of Malpighian tubes ; the hind 

 gut ends in a ventral anus beneath the base of the sting. 

 The narrowness of the gut may be associated with the fluid 

 nature of the food. 



The body cavity is for the most part filled up with organs, 

 muscles, and connective tissue. A pair of coxal glands, per- 

 haps excretory and nephridial, but apparently closed in the 

 adult, lie near the base of the last two walking legs. 



The blood contains amoeboid corpuscles and the respira- 

 tory pigment haemocyanin. An eight-chambered heart, 

 within a pericardium, lies along the back of the mesosoma. 

 It gives off lateral arteries from the posterior end of each of 

 its chambers, is continued backwards in a posterior aorta, 

 and forwards in an anterior aorta. The latter supplies the 

 head, and divides into two branches encircling the gullet 

 and reuniting in a ventral artery above the nerve cord. From 

 capillaries the blood is gathered into a ventral venous sinus, 

 is purified in the lung books, and thence returns by veins to 

 the pericardium, finding its way by valved lateral openings 

 (ostia) into the anterior end of each heart chamber. 



On the ninth to twelfth segments lie slit-like stigmata, the 

 openings of four pairs of lung books. Each lung book is 

 like a little purse with numerous (over a hundred) compart- 

 ments. Air fills the much-divided cavity, and blood circul- 

 ates in the lamellae, which form the partitions. These lung 

 books or pulmonary sacs are believed by some to be 

 chambered or plaited tracheae, while Professor Ray Lankes- 

 ter regards them as invaginated modifications of gill books 

 such as Limulus possesses. 



The testes consist of two pairs of longitudinal tubes, 

 united by cross bridges ; the vas deferens, with a terminal 

 copulatory modification, opens under the operculum on the 

 first abdominal segment. The ovary consists of three longi- 

 tudinal tubes, united by cross ducts, and two oviducts open 

 on the under surface of the genital operculum. 



Fertilisation is internal ; the ova begin their development 

 in the ovary, and complete it in the oviduct. The segmenta- 

 tion is discoidal, the ova are hatched within the mother. 

 The young, thus born " viviparously," are like miniature 

 adults, and adhere for some time after birth to the body of 

 the mother. 



