QUESTIONS ON THE CCELENTERATA. 259 



24. What genera of Ccelenterata are known to inhabit fresh water? 



25. Give some account of the geographical distribution of the Sertu- 



laridas. 



26. Describe the minute structure of the body- wall in Actinia. 



27. What law appears to determine the number of parts among the 



several orders of Actinozoa ? 



28. What is the number and structure of the tentacles in the 



Alcyonaria ? 



29. Describe the structure of a typical ' corallite.' 



30. Define the order Rugosa. 



31. Explain the formation of the gyrate corallum of Moeandrina. 



32. How does a < sclerobasis ' differ from a true corallum ? 



33. Compare the nutrient system in 



a. Actinia and Pleurobrachia. 



b. Pleurobrachia and Beroe. 



34. Describe the structural relations of the tentacles in 



a. Actinia; 



b. Cestum ; 



c. Pleurobrachia. 



35. What characters distinguish the thread-cells of the Cteno- 



phora ? 



36. Describe the structure and position of the < ctenocyst.* 



37. Give some account of the nervous system of the Ctenophora. 



38. What peculiarity of position distinguishes the reproductive 



organs of Tubipora ? 



39. Define precisely the position of the male and female organs in 



the Ctenophora. 



40. What, according to Haime, is the number and succession of the 



tentacles in the common Sea-anemone ? 



41. Give some account of the development of the canal system in 



Beroe. 



42. What numerical law governs the development of the < septa ' in 



a Zoantharian corallite ? 



43- Distinguish three principal modes of gemmation among the co- 



ralligenous Actinozoa. 



44- Describe the structure of a Fringing-reef. 



45- How has Mr. Darwin explained the true nature of Barrier-reefs 



and Atolls ? 



S 2 



