GENERATIVE ORGANS OF ARTHROPOD A. 291 



united by lateral longitudinal trunks. As however they have only 

 three pairs of stigmata they indicate their affinity to the other 

 divisions of the Arachnida. A remarkable peculiarity is found in 

 their tracheal system; a tracheal trunk arises from a stigma and 

 speedily breaks up into a large number of short lamella-like flattened 

 branches, which lie on one another like the leaves of a book ; owing 

 to this arrangement the whole organ is confined to a small space. 

 These leaf-like trachea? are known as " Lungs." Four pairs of 

 them open on to the ventral surface of the abdomen in the Scorpionea. 

 There are two pairs in the Phrynida and Mygalida. In the rest of 

 the Aranea one pair only is developed, the stigmata of which lie in 

 the anterior portion, and on the ventral face of, the abdomen. In 

 some Aranea a second pair of stigmata, placed just behind the first, 

 lead into trachea} which end by two chief tubes, at the end of which 

 are extremely fine tubules (Argyroneta, Dysdera, Segestria). In 

 others this pair of stigmata is fused, and lies in front of the spinning- 

 warts. Four tubes generally pass from the stigmatic cavity; these 

 are either branched (Thomisus), or end simply (Tegeneria, Clubiona, 

 Lycosa, Epeira). In the absence of branches and of anastomoses, 

 the leaf-like trachea3 resemble the lowest stage of the trachea?, 

 and represent a special development of them. 



The Opilionida, the trachea? of which are distinguished by the 

 large number of their branches, have only one pair of stigmata. 

 The number is also reduced in the Acarina, in many of which (e.g. 

 Sarcoptes) there is no tracheal system at all, as is the case also in 

 the Linguatulida and Pycnogonida. 



Generative Organs. 

 § 226. 



Reproduction in the Arthropoda is effected solely by the genera- 

 tive system ; all the modes of reproduction in them, which are called 

 asexual (phaanomena of Parthenogenesis, and of alternation of 

 generation), are due to sexual differentiation. 



The disposition of the generative organs iu different individuals, 

 which obtains in some divisions only of the Vermes, is the rule in 

 the Arthropoda; in a few only is the hermaphrodite structure 

 retained. In many, sexual differentiation extends even to the outer 

 parts of the body, to its size and form. 



The germ-glands are always distinct organs, which are never 

 distributed in a metameric manner ; they are either single, or there is 

 but one pair of them. We cannot yet say whether this is due to 

 the generative system having been transmitted from animals in 

 which it was single. 



In the general arrangement of the organs there are many constant 

 and very remarkable points. The typical form of the apparatus is 



u 2 



