viii PHYLUM MOLLUSCOIDA 319 



which the organs of the larva become merged. The outer layer of 

 the sac is in continuity with the mesoderm layer of the body-wall 

 and gives rise to the mesodermal parts of the zooid. The inner 

 layer, derived from the ectoderm, forms the deric epithelium, the 

 nervous system, and the entire enteric epithelium. A diverticulum 

 of the sac constitutes the first rudiment of the stomach and 

 intestine ; a second diverticulum forms the rudiment of the 

 oesophagus ; these become applied to one another and fuse to 

 form the continuous alimentary canal. The ganglion arises as an 

 invagination of the ectoderm in the space between the mouth and 

 the anus. The upper part of the cavity of the primitive sac, after 

 the rudiment of the alimentary canal has become separated off, 

 forms a space termed the atrium ; the walls of this become con- 

 verted into the tentacle sheath, while on its base appear the rudi- 

 ments of the tentacles and lophophore. During the development 

 of the organs of the adult zooid the brown body becomes closely 

 applied to the stomach and gradually absorbed. 



The primary zooid thus formed gives rise asexually by a process 

 of repeated budding to the branching structure which has been 

 described. In many of the zooacia of a fully-developed colony no 

 zooid is found to be present, but instead there is a dark brown 

 body similar to that which occurs in the primary zooecium. These 

 are zooids that have undergone degeneration - - the lophophore, 

 tentacles, and alimentary canal having become absorbed. Such 

 degenerated zooids are capable of regeneration, the organs becoming 

 re-developed and the brown body re-absorbed. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Polyzoa are Molluscoida which, with one exception, form 

 colonies of zooids connected together by a common organic sub- 

 stance. There is a lophophore bearing a series of slender, ciliated, 

 post-oral tentacles. The anterior part of the body forms, in the 

 majority, a short introvert, within which the lophophore and the 

 tentacles are capable of being withdrawn. In some the pro- 

 stomium is represented by a small lobe the epistome. The 

 alimentary canal is U-shaped, and the anus is anterior, within, or 

 just outside of, the tentacular circlet. In most the nervous system 

 is represented only by a small ganglion between the mouth and 

 the anus. A cuticle, sometimes gelatinous, sometimes horny, 

 sometimes calcified, forms a firm exoskeletal layer for the support 

 of the colony. Nephridia (corresponding to the head-nephridia of 

 the Trochosphere) occur only in the Endoproda. There is no 

 vascular system. The sexes are usually united. The majority of 

 Polyzoa occur in the sea; a limited n timber are inhabitants of 

 fresh water. 



