THE CHJITOGNATHA, OE PRIMITIVE MOLLUSCA. 385 



1. The orig-inal bilateral symmetry of tlie Mollusca is 

 presented by the Chictognatlia in its most perfect form, 

 especially in respect of the body cavities. 



2. The Chastoguatha resemble many Mollusca of undoubt- 

 edly primitive type, in the absence of apparent segmenta- 

 tion. 



3. The vermiform shape of the body, recalling that of the 

 Amphinenra aplacophora. 



4. No extensive hasmococl has been hitherto identified. 



5. The alimentary canal is straight ; the anus opens in 

 front of part of the visceral sac. 



6. There is no evidence of a radula, either in the Cha3to- 

 gnatha or in their ancestors. The buccal armature is other- 

 wise very like that of many Mollusca. 



7. The nervous system is of the Molluscan type. 



8. The "hood," the suggested homologue of the circum- 

 oral Cephalopod '' foot." 



9. The growth of the genital cells within a follicular 

 epithelium and upon stalks. Hermaphroditism. 



10. The two pairs of openings from the perigonadial 

 coelom to the exterior. These are believed to be the homo- 

 loguesof the two pairs of ducts leading from the pericardial- 

 gonadial coelom of the more primitive Mollusca. 



11. The cephalic endo-skeleton and lateral fins. 



12. The preoral ciliated ring ; the suggested homologue of 

 the velum. 



The characters of the Chsetognatha are, on the whole, 

 just those of the more archiac types of Mollusca rather than 

 of the Grastropoda or Lamellibranchia. Their affinity to the 

 Aplacophoran Amphineura is indicated by the vermiform 

 shape and bilateral symmetry of the body, by the straight 

 alimentary canal, by many negative characters, such as 

 absence of shell, foot, and radula. The Amphineura may 

 have a more primitive nervous system, but in the Chteto- 

 gnatlia the paired arrangement of body cavities is the more 

 primitive. Molluscan nephridia and Cha3tognath sperm-ducts 

 would appear to be homologous structures, but the genital 



