236 



PHYLUM ANNELIDA. 



The development is in most cases insufficiently known, and it is 

 probable that further knowledge of it will remove these sets of animals 

 from their apparently anomalous position. 



Class PHORONOIDEA. 



This class has been erected for the single genus Phoronis, which has 



been associated both with the Gephyrea 

 and with Polyzoa. With the removal of 

 Cephalodiscns and Rhabdopleura from the 

 last-named group to the Hemichorda, 

 Phoronis has been left in a somewhat 

 isolated position. Recently it has been 

 proposed by Dr. Masterman to re- 

 associate it with these forms and with 

 Balanoglossus, on account of certain 

 Chordate affinities said to be exhibited by 

 the larva. The point will be further 

 discussed in the chapter on Balano- 

 glossus. 



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 parti- 

 cles. Each individual is furnished with 

 a horseshoe-shaped crown of tentacles, 

 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 red cells. The body 

 cavity is well developed, and is divided 

 into chambers. The sexes are united ; and 

 the larva, known as Actinotrocha^ under- 

 goes a remarkable metamorphosis in the 

 course of its conversion into the adult. 



CNR 



FIG. 112. Actinotrocha or 

 larva of Phoronis. After 

 Masterman. 



The mouth is overhung by the 

 prominent pre-oral hood ; the 

 anus is at the other end of 

 the body. Behind the mouth 

 is a ring of ciliated tentacles. 



SP., the nerve ganglion in the 

 hood ; NG., the nerve gan- 

 glion of the region called collar 

 region by Masterman ; CNR., 

 nerve-ring at base of tentacles. 



Class POLYZOA. 



As usually defined, the class includes 

 two sub-classes, the Ectoprocta and the 

 Entoprocta, but it seems almost certain that these are distinct classes. 



The Ectoprocta include fresh-water and marine forms, in which the 

 anus is outside the basis of the tentacles. The nervous system is 

 represented by a ganglion placed between the mouth and anus. There 

 is no vascular system. Nephridia are absent. All are colonial and bud 

 very freely ; the marine forms show considerable division of labour 

 among the members of the colony. 



