72 INVERTEBRATE ZOOLOGY. 



are bisexual and produce eggs or spermatazoa. These unite in the water and 

 develop into a polyp which may develop asexnally into another colony. 4) 

 There are also minor differences in the shape and arrangement of the tenta- 

 cles, the nettle cells, the stem,- etc. 



37. Tliamnocnidia (or Clava). These are solitary polyps, although gen- 

 erally associated in chimps. Each polyp bears at its base very niimerous 

 mediisa-buds or gonophores in racemose clusters. The male gonophores libe- 

 rate the spermatozoa, which escape into the water : biit the female gonophores 

 retain the eggs which are fertilized and develop in situ, and leave it in the 

 form of minute polyps, which pass out backwards, drawing their tentacles 

 after them. Cross-fertilization is insured because a given polyp produces 

 gonophores of only one sex ; the polyp, although asexual, is thus often termed 

 male or female. Tliamnocnidia is too large for mounting entire. It may be 

 studied in a watch crystal, and separate gonophore-clusters mounted. 



Clava may be mounted by cutting little strips of cardboard, soaking them 

 a few minutes in turpentine and placing them in the balsam with the speci- 

 men, and in such a way that the weight of the cover glass is sustained by 

 them. 



ScYPHOZOA and Ctenophora. 



Members of these classes are peciiliarly difficult of preservation and can be 

 stiidied well only at the sea-shore. The hard parts of coral can be studied in 

 any good collection. The spicules of Alcyonaria (octocoraUa) are microscopic, 

 and may be isolated as follows : — 



38. Place in a test-tube a small portion of the Polyparixim of any 

 Alcyonarian. Cover with K-O-H and boil until there is left a residue in the 

 bottom of the tube. Axial portions or other hard masses may be removed 

 and the residue handled by decantaticni. Allow the residue to settle and then 

 pour off the excess of K-O-H. Fill up with water, shake gently, let it settle 

 and again decant the excess. Repeat the washing with water two or three 

 times and then wash successively in 10%, 95%, 100^, and finally "turpentine, 

 leaving the material in the last three for some hours each. Pipette a drop of 

 the material thus prepared upon a slide, drain off the excess of turpentine 

 with a cloth or blotting-paper, add balsam, cover and examine. Different 

 species will yield different results. [This experiment will furnish material 

 sufficient for an entire class.] 



