INVERTEBRATE ZOOLOGY. 11 



Intestine incomplete and without anal orifice. Protocoelom secondarily filled 

 up by a connective tissue parenchyma, derived from the mesenchyma. Pro- 

 tonephridium a system of branching tubules, the so-called "water-vascular 

 system." Mostly hermaphroditic with complicated reproductive system. 



Cladus II. Vermes. 



Forms derived from Trochozoon. Protocoelom serves as body cavity and is 

 not filled with mesenchyma. Intestine complete with anus. The animals of 

 this class are externally very unlike and fitted for many different environ- 

 ments. Some are plainly modified Trochophores when adult, others pass this 

 stage in early life, while in others the stage has dropped out. They are most 

 easily distinguished from other worm-like forms by their lack of segmentation. 



Sub-type II. Met.\nephridozoa {Aposeolecida). 



Anterior part of body alone {Prosoma) derived from Trochozoon, to which 

 the remainder {^Metasoma, generally the larger portion) is added by success- 

 ive growths forming segments or somites. Each somite forms organs like the 

 l^rosoma, which are distinguished as meta-iiephridia, and .secondary gonads. 

 In some forms the organs of the prosoma remain to supply that portion, in 

 others they early disappear, the region being secondarily supplied by in- 

 growths from the metasomatic organs. The body-cavity is the meta-coelom, 

 formed by paired mesodermic coelom sacks, which may be considered as ex- 

 pansions of the sack gonads in which the germ cells are localized in certain 

 areas as "surface gonads." The metanephridia communicate internally with 

 the coelom and externally with the outside. They may be repetitions or seg- 

 mentations of the protonephridia, secondarily connected with the coelom ; or 

 they may be evaginations of the coelom itself. 



Cladus I. Articulata. 



Segmentation very pronounced, each segment typically furnished with a 

 pair of appendages, jointed in the higher forms. Segments and appendages 

 often show marked specialization in different body regions. Integument pro- 

 tected by a cuticula which may become a thick chitinous exoskeleton. Meta- 

 coelom spacious, mesenchyma poorly developed : nervous system a double 

 cerebral ganglion dorsal to intestine, connected by commissures to a ventral 

 ganglionic chain. Metanei)hridia in the typical form segmentally arranged, 

 in higher forms modified or fail, being replaced by other organs. 



