CUUSTACEA. 411 



Crustacea, there is such a marked sexual dimorphism that the males 

 remain small and dwarfed, and are attached like parasites to the 

 body of the female. During the act of copulation, which is often 

 limited to the external union of the two sexes, the spermatophores 

 are fastened to the female genital segment or thrust into the vagina 

 by the organ of copulation, whence they sometimes pass into a 

 special receptaculurn. seminis. Most Arthropoda are oviparous, but 

 in almost every group there are viviparous forms. The eggs are 

 frequently carried about by the mother, or deposited in protected 

 places where food may easily be obtained. The embryonic development 

 (i.e., development within the egg) is characterised, except in the case 

 of the small stout embryos of the Cyclopidce, Pentaslomidce and 

 Acarina, by the presence of a ventrally placed primitive streak, from 

 which especially the ganglionic chain and the ventral parts of the 

 segments proceed. The more or less complex embryonic development 

 is usually followed by a complicated metamorphosis, during which 

 the young form as larva undergoes several ecdyses. Numerous seg- 

 ments and parts present in the adult are not unfrequently wanting in 

 the just-hatched larva ; in other cases, all the segments of the adult 

 are indeed present, but are not as yet fused together to form regions. 

 In such cases, the larvae resemble the Annelida in their homonomous 

 segmentation, and in their locomotion and mode of life. The meta- 

 morphosis may however be retrogressive ; the larvse are hatched with 

 sense organs and appendages, but in the further course of develop- 

 ment they become parasitic, lose their eyes and organs of locomotion, 

 and develop into strange unsegmented (Lernceos) or entozoon-like 

 (Pentastomidce) forms. 



The Arthropoda are no exception to the general rule that the 

 aquatic forms which breathe by gills are lower and, from a genetic 

 point of view, older than the air-breathing members of the same 

 group, inasmuch as the Branchiata or Crustacea are the older, the 

 Tracheata the younger types. 



CLASS I.- CRUSTACEA.* 



Aquatic Arthropoda, ii'hich breathe by means of gills. Thejf Jiave 

 two pairs of antennce ; numerous paired legs on the thorax, and 

 usually also on the abdomen. 



* Milne Edwards. ' Histoire naturelle dcs Crustaces," 3 vol. and atlas. 1S3S- 

 1840. C. Glaus, " Untersuchuncren zur Erforschung dcr genealogischeii Grund- 

 laje des Crustaccensystems," Wien, 1S7G. 



