248 



PHYLUM ANNELIDA. 



Appendix (4) to Annelid Series 



Under the old term Molluscoidea are sometimes included the three 

 classes Phoronoidea, Polyzoa or Bryozoa, and Brachiopoda. 



The Molluscoidea are characterised by the presence of a true 

 coelom, 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 mouth is typically furnished with ciliated tentacles, and is often 

 overhung by an epistome ; both tentacles and epistome, when present, 

 contain spaces which are part of the body cavity. Except in the 

 Ectoprocta among Polyzoa, two or four nephridia are present, and 

 serve also as genital ducts. There is always a metamorphosis in 

 development, and the larvae are peculiar. 



The development is in most cases in- 

 sufficiently known, and it is probable 

 that further knowledge of it will remove 

 these sets of animals from their apparently 

 anomalous position. 



Class PHORONOIDEA. 



CNR 



FIG. 127. Actinolrocha or 

 larva of Phoronis. After 

 Masterman. 



The mouth is overhung by the 

 prominent pre-oral hood ; the 

 anus is at the other endof^the 

 body. Behind the mouth is a 

 ring of ciliated tentacles. 



SP., the nerve ganglion in the 

 hood ; N.G. t the nerve gan- 

 glion of the region called collar 

 region by Masterman ; CNR., 

 nerve-ring at base of tentacles. 



This class was erected for the genus 

 Phoronis^ which has been associated both 

 with the Gephyrea and with Polyzoa. 

 Another genus, Pkoronopsts, from the 

 Cape, has been recently established. It 

 has been proposed to associate these 

 two genera, along with Cephalodiscus 

 and Rhabdopleura,) with the Hemi- 

 chorda, on account of certain Chordate 

 affinities, said to be exhibited by the 

 larva. 



The genus Phoronis includes a few 

 species of small marine " worms," social 

 in habit, and found enclosed in fixed 

 leathery tubes often encrusted with foreign 

 particles. Each individual is furnished 

 with a horseshoe-shaped crown of tent- 

 acles, which are hollow and supported 

 by an internal skeleton. The nervous 

 system lies in the ectoderm a very 

 primitive character, and consists of a 

 ring round the mouth, and of a cord 

 down the left side of the body. An 

 interesting point is the presence of a 

 closed vascular system with nucleated 



