BRANCH MOLLUSCOIDA 



In this branch there is usually a hotly cavity, with the ah- 

 mentary tube suspended by mesenteries. The mouth and anal 

 aperture are near together, the dorsal surface being shortened. 

 In the atlult there is a tentacle-bearing ridge, or lophophore, about 

 the mouth, containing a compartment of the body cavity. 

 The tentacles are used not only in securing food, but in respira- 

 tion. The nervous system consists of one or two ganglia or of a 

 nerve ring.^ 



CLASS I. POLYZOA 



MoUuscoi'da, which usually form colonies of zooids by budding, are 

 Pdlyzf/a.^ The character of the colony differs, according to the mode of 

 budding in the different species and the character of the exoskeleton. It 

 varies from a bush-like colony to a calcareous or gelatinous sheet. Each 

 zooid has a crown of ciliated tentacles which can be extended or with- 

 drawn. They are held together by the common exoskeleton formed by 

 the ectoderm. There is no vascular system. The digestive tract is bent 

 like the letter U, the anal opening being near the mouth, within or just 

 outside of the ring of tentacles. The nervous system consists of a gang- 

 lion situated between the mouth and the anal opening. Polyzoans are 

 usually hermaphroditic. 



CLASS n. PHORONI'DA 



The classification of this group of worm-like forms of the sea is doubtful. 



The worm is covered by a leathery cylindric tube into which it may 

 withdraw. The body is unsegmented and bears a crown of tentacles. 

 The mouth and anus are close together and are situated at the tentacle- 

 bearing end of the body. The body cavity is divided into three chambers. 

 There is an alimentary tract and a closed system of blood-vessels contain- 

 ing red blood-corpuscles. The central nervous system consists of a horse- 

 shoe-shaped nerve ring at the base of the tentacles. The Phoronis is 

 hermaphroditic. There is a metamorphosis. 



CLASS m. BRACHIOP'ODA 



Brachiopods are marine and were abundant in former geologic times, 

 being very plentiful as early as the Cambrian Period. There are a few 

 living species. 



They are enclosed in a bivalve shell (Fig. 38), the valves being dorsal 

 and ventral instead of right and left, as in the mollusks. They are at- 

 tached to foreign objects by a -peduncle or stalk, which passes through the 

 larger or ventral valve near the hinge. They do not form colonies. 

 1 Parker and Haswell, p. 313. 

 48 



