396 LECTURE XVIII. 



on the fourth and seventh primary segments, though, in the male, 

 they be near the anal end of the body in the nine-jointed larva, 

 become advanced much nearer the head in the fully developed 

 lulus, by reason of the vast superaddition of joints, through the 

 successive sextuple gemmation of segments between the penulti- 

 mate and antepenultimate primary segments of the larva. In the 

 chilopoda, the ordinary insect-type of the generative apparatus vi^as 

 more nearly approached, by not only the more definite boundary 

 between testis and vas deferens, but by the termination of the 

 sperm-ducts at the anal segment, and, likewise, by the presence of 

 accessory glandular organs. 



Testes with distinct sperm-ducts and superadded glands are 

 present in all hexapod insects ; and in all, with a few exceptions, the 

 sperm-ducts open at the base of an intromittent organ, developed 

 from the anal segment. 



The testes are remarkable for the endless diversity of their forms, 

 and often for the bright or brilliant-coloured pigment which besets 

 the tunica vaginalis ; in both characters reminding us of the flowers 

 of plants. They appear to form a single organ in most Lepidoptera, 

 but are actually two confluent testes, and were originally distinct in 

 the larva of all that beautiful order ; that distinction being indicated 

 by a circular groove, and still more significantly by the two distinct 

 sperm-ducts. 



In most insects the testes form a distinct pair of glands ; but their 

 ca3cal structure, and the gradational development of the secerning 

 follicles, at length produce a seeming multiplication of testes, and it 

 is difficult to avoid giving this definition to the six clusters of sperm- 

 atic casca, with their six ducts, on each side in the dung-beetle, 

 Scarabmis^ or to the twelve flattened circular glands, with as many 

 ducts, which represent the testis on each side in the rose-beetle, Cetonia. 

 In all these cases, however, the ducts from the divisions or distinct 

 lobes of the testis rapidly unite to form the beginning of a single vas 

 deferens on each side ; and the essentially dual character of the 

 testes is manifested by the pair of vasa deferentia, whether the cha- 

 racter be masked in the gland itself by confluence, as in the butter- 

 flies, or by multifid division, as in the beetles. 



In the Aptera, Treviranus* has given a good description and 

 figure of the male organs of the Lepisma. The testes are repre- 

 sented, on each side, by four or five elliptical glands, the slender 

 ducts of which, after some irregular ramifications, communicate with 

 a common vas deferens, which, after a long fold, descends and dilates 



* CCLVI. ii. p. 15. taf. iv. fijr. 2. 



