cxxii Introduction. 



anus^ except in Daphiidae ; but not always on numerically corre- 

 sponding- teguments in tlie two sexes. Crustacea attain the power 

 of sexual reproduction before they attain their full size, and they 

 retain it, and repeatedly exercise it, in many cases at least, for an 

 indefinite period. 



In development the yolk may undergo complete segmentation, 

 and a larva of the form known as Nauplius may be produced with- 

 out the formation of any primitive streak, as is the case with the 

 Cirripedia, Entomostraca, Penaeus amongst Decapoda, and most 

 BrancMopoda ; or, as in all the higher Crustacea, the yolk under- 

 goes a partial segmentation, and the embryo, when ready to be set 

 free from the egg, may either be of the larval form known as Zoea, 

 as in most Decapoda ; or may differ from the adult form only by 

 the possession of certain structures, which are subsequently aborted, 

 as is the case with the outer division of the abdominal limbs of the 

 common lobster, Homarus vulgaris ; or, finally, may, as is the case 

 with Astacus fluviatilis, from the time it is set free from the egg, 

 possess the same number and proportions of appendages as the 

 adult. Instances of retrograde metamorphosis are common in this 

 class, several of the orders of which possess parasitic families. Par- 

 thenogenesis occurs in the Cladocera, the ' summer eggs^ o^ Daphnia 

 being developed without sexual congress, and one genus, Cytliere, of 

 the allied family, Ostracodea, is viviparous. 



Sub-kingdom, Vermes. 



Bilaterally symmetrical animals, very various in shape and other 

 external characters, but agreeing in the absence of heteronomy 

 from their post-cephalic regions, in not possessing hollow seg- 

 mented limbs, and in having their locomotor muscles closely con- 

 nected with their integumentary system, not only on the ventral, 

 but also on the dorsal and lateral aspects of their body walls. 



The Sub-kingdom Vermes consists of two Divisions, the Annulata 

 and the Annuloida. The first of these contains the multisegmeutal 

 Vermes, which, in Mr. Herbert Spencer's language, would be spoken 

 of as ' aggregates of the third order.^ Their bodies are more or less 



