224 SEGMENTED WORMS OR ANNELIDA. 



that some of them may become of great interest to the systematic 

 zoologist, but we do not yet understand what places in the system they 

 should occupy. Moreover, i^s they are small, unfamiliar, and unknown 

 to myself, I shall simply refer the curious to what more complete works 

 say about the Gasterotricha, Echinoderidte, Demoscolecidce, and 

 Chietosomidte. 



Appendix (3) to Annelid Series. 



Class SiPUNCULOiDEA, €.g.^ Sipuncuhis. 



Marine worms usually living in the sand. The body is elongated and 

 apparently unsegmented. The oral or anterior region can be invaginated 

 by special muscles. There are no setre. They are sometimes, but 

 perhaps erroneously, placed beside Echiuridce as Gephyrea Achista. The 

 nervous system consists of an oesophageal ring, and a median ventral 

 nerve cord, which shows slight hints of segmentation. There is a 

 spacious body cavity. 



SlFUNCULlD>E. 



Pkiapulid.^. 



J ^ 



The anus is dorsal and anterior, and The alimentary' canal is straight or 



the food canal is usually in a spiral : the ! slightly looped, and the anus is dorsal 



mouth is surrounded by tentacles. There 

 is a closed vascular system, with branches 

 to the tentacles. 



An anterior pair of nephridta serve also 

 as genital ducts, removing the repro- 

 ductive cells from the body cavity. The 

 sexes are separate. 



Examples — Sipunctihis. 



Pkascolosoiiia. 



ind posterior. There are no tentacles. 



There is no vascular system. 



No anterior nephridia, but a pair of 

 tubes open beside the anus, and are said 

 to be excretory in the young, genital in 

 the adult. The sexes are separate. 



Ex am pi es^-/"? -/a/ // ins. 



Halicryptus, 



Appendix (4) to Anitelid Series. 



Under the old term MoUuscoidea are sometimes included the three 

 classes^Phoronoidea, Polyzoa or Bryozoa, and Brachiopoda. Prof. 

 Lang includes them along with Sipunculoids in the provisional group 

 Prosopygii. 



The MoUuscoidea are characterised by the presence of a true co?lome, 

 formed in development by the folding off of pouches from the 

 archenteron, and by the shortening of the dorsal region of the body, 

 which results in the close approximation of mouth and anus. The 

 moulh is typically furnished with ciliated tentacles, and is often over- 

 hung by an epistome ; both tentacles and epistome, when present, contain 

 spaces which are part of the body cavity. Except in Polyzoa, two or 

 four nephridia are present, and serve also as genital ducts. There is 

 always a metamorphosis in development, and the larvce are peculiar. 



Class Phoronoidea. 



The crown of tentacles is shaped like a horse shoe, each tentacle is 

 supported by an internal skeleton. The nervous system lies in the 



