PHYLUM MOLLU SCOT DBA 161 



aboral extremity of the zooid. In addition to the budding which 

 gives rise to the colony formation, sexual reproduction may also 

 take place. Most ectoprocts are hermaphroditic. Ovaries 

 and spermaries make their appearance either in the lining of 

 the coelom or in the tissues of the funiculus. The gonads dehisce 

 into the coelom where fertilization takes place. In some species 

 special chambers, called ovicells, are provided for containing 

 the developing embryos. Following cleavage, a free-swimming 

 larva of variable form in different species makes its appearance. 

 This larva undergoes a transformation to form a zooid from which 

 a colony later develops by budding. Statoblasts, or internal 

 buds, are characteristic of many fresh-water ectoprocts. These 

 are surrounded by chambers which upon drying become filled 

 with air and serve to float the statoblasts. In the fresh-water 

 genera, Pectinatella and Cristatella, the free larval stage has 

 been retained. 



II. Order Endoprocta 



In members of this order, the anal opening occurs within the 

 circle of tentacles. With the exception of one genus, Urnatella, 

 the entire group is marine. The body is usually cup-shaped, 

 enclosing at its open end a cavity called the vestibule (Fig. 80-4), 

 which contains the mouth and the anus. The rim of this 

 cavity bears the tentacles, which are capable of being withdrawn 

 into the vestibule. The space between the alimentary canal and 

 the body wall is filled with a gelatinous matrix. This lack of an 

 undisputed coelom, together with the fact that the excretory 

 system is protonephridial, furnishes ground for doubting any close 

 relationship between the Endoprocta and the Ectoprocta. 

 Pedicellina, Urnatella, Loxosoma are characteristic genera. 



The larva formed by the endoprocts undergoes a less con- 

 spicuous transformation than that of the ectoprocts. After 

 becoming fixed, it transforms into a zooid from which other indi- 

 viduals arise by budding. 



Class Brachiopoda 



Because of the presence of a bivalve shell, brachiopods are 

 frequently confused with molluscs. The shell of the brachiopod 

 is, however, composed of a dorsal and a ventral valve (Fig. 81), 

 while the valves of the bivalve molluscs are lateral. In the 

 brachiopods, the valves are articulated in their posterior regions 



