TEST QUESTIONS FOR STUDENTS 917 



resemblance to an earthworm, Lumbricus, in shape and size. What 

 external features are obviously very different ? [Unsegmented, without 

 pigmentation, with terminal mouth, without setae, etc.] 



22. Explain what is meant by a typical coelom or body-cavity. 

 [The space between the gut and the body-wall, hned with mesoderm, 

 often containing a perivisceral fluid, shut off from the blood system, 

 with the gonads on its wall, and often communicating with the exterior 

 by means of nephridia.J 



23. What new structural features make their first appearance in 

 worm-like animal? ? [Definitely bilateral symmetry, head-brains, 

 definite mesoderm, a coelom.] 



24. Contrast the mode of nutrition in a tapeworm with that in a 

 round worm. [The tapeworm, without mouth or food-canal, absorbs 

 fluid food by its whole surface. The round worm, with a mouth and 

 food -canal, ingests its food in the usual way.] 



25. Mention four good examples of regenerative capacity among 

 animals. What general statement can be made as to its occurrence ? 

 [Hydra can regrow its tentacles, a starfish a lost arm, a snail its horn 

 with the eye at the tip. and a hzard its tail. Regeneration tends to 

 occur in those animals and in those parts of animals which in the 

 natural conditions of hfe are peculiarly Hable to non-fatal injury.] 



26. Draw and describe a kind of ceU characteristic of (a) all sponges, 

 {b) of calcareous sponges, (c) of almost all Coelentera, and {d) of Plathel- 

 minthes. [(a) Choanocyte, (6) porocyte, (c) cnidoblast, {d) flame-cell.] 



27. A decapitated earthworm can re-grow a head, but a decapitated 

 leech dies. Why is there this difference ? 



28. Describe \n detail the alimentary system of the earthworm, and 

 explain exactly what happens in each region. Do the .same for other 

 types, such as' crayfish, cockroach, dogfish, frog, pigeon, rabbit. 



29. Sum up the agricultural importance of earthworms. [Burrowing, 

 making way for plant-roots, rain-drops, and air ; bruising, grinding the 

 soil in the gizzard ; burying the surface with castings brought up from 

 beneath and also burying leaves.] 



30. Explain what is meant by the following : — blastula or blasto- 

 sphere, morula, blastoderm, gastrula. blastopore, archenteron. 



31. Make a tabulated survey of the four chief modes of ovum- 

 segmentation, and give two examples of each. 



32. What is implied in the fertilisation of an animal ovum ? 

 [(i) Minghng of maternal and paternal inheritances, probably for 

 the most part in the chromosomes of the nuclei. (2) Restoring the 

 reduced number of chromosomes to the normal. (3) Introducing (in 

 the middle piece of the spermatozoon) a centrosome which plays an 

 important part in the segmentation of the ovum. (4) Stimulating the 

 ovum to divide or removing some inhibiting factor. (5) Causing a 

 rapid chemical and physical change in the periphery of the ovum which 

 " blocks " it, or makes it non-receptive to other spermatozoa.] 



33. What happens to a typical ovum immediately before and im- 

 mediately after fertilisation ? [Maturation before, segmentation or 

 cleavage after.] 



34. What is meant by a nerve ganglion ? Give an elementary 

 account of its chief functions. 



