ARTHROPODA. 495 



Arthropoda are very rarely hermaphrodite. The male is generally 

 distinguished from the female by differences of size, colour, by the large 

 development of special sense organs, or of copulatory and prehensile 

 organs. The form of the testis and ovary is subject to much variation. 

 The glands are typically paired, but the members of the pair may be fused. 

 The ducts are also paired, but may fuse distally, and the sexual apertures 

 are accordingly paired or single. They are placed in the thorax or 

 abdomen, and are sometimes terminal. Accessory organs are frequently 

 present, but vary in the different classes. The spermatozoa are commonly 

 united into spermatophores by the secretion of the walls of the vasa 

 deferentia, or of special glands. The ova vary in size. Secondary yolk is 

 generally present, and is placed centrally (centro-lecithal ova). When the 

 ovum segments, fission is consequently on the centro-lecithal type, and 

 either regular, unequal, or superficial. But there are exceptions. Regular 

 segmentation, with completely separate blastomeres, occurs in Podura 

 among Insecta, and in the lower Crustacea, rarely in the higher. Telo- 

 lecithal ova, with partial segmentation, are found in some Crustacea (para- 

 sitic Copepoda, Isopoda, Mysis) and in the Arachnid Scorpio, but it is 

 possible that they are modified from the centro-lecithal type. In develop- 

 ment, certain differences are observable between the Crustacea and tracheate 

 Arthropoda, which make it possible that they may form two distinct phyla, 

 or branches of a phylum which separated at a very early period (?). In the 

 Crustacea the archenteron is usually and primitively invaginate ; the meso- 

 blast originates from the wall of the invagination, and forms a layer 

 between the epi- and hypo-blast ; the proctodaeum is formed before the 

 stomodaeum, and these two parts are usually of great relative length as 

 compared with the mesenteron. In tracheate Arthropoda the archenteron 

 is not invaginated, and the mesenteron is derived from the yolk-cells ; the 

 mesoblast developes as a median thickening of the ventral plate, which 

 divides into two bands. These bands in Spiders and Myriapoda, and 

 perhaps in Insecta, divide into somites, with cavities continued into the 

 limbs. The stomodaeum developes before the proctodaeum, and the 

 mesenteron is relatively large. The proctodaeum gives origin to the 

 Malpighian tubes. In the tracheate Arthropoda and higher Crustacea, a 

 thickening of the blastoderm gives rise to a ventral plate, by the growth of 

 which the body is subsequently formed. In the Insecta, and perhaps 

 Myriapoda, this plate is marked by a median longitudinal mesoblastic 

 groove, or by a keel-like thickening in Spiders. Groove and thickening 

 alike are generally termed 'primitive streak,' and, like the structure so- 

 called in Vertebrata, are probably the remnants of a blastopore. 



Parthenogenesis occurs in some Insecta and Crustacea, and leads to a 

 form of Alternation of Generations known as Heterogamy. Sexual 

 dimorphism is very common in the two classes named. Some Crustacea 



