522 



POLYZOA 



stained or unstained ; and are essential for the examination of 

 the softer forms. In the case of erect species, both surfaces of 

 the branch should he looked at. The opercula, avicularia, and 

 rosette-plates afford important systematic characters in the case 

 of the Cheilostomata. 



It must not be forgotten to take account of the condition of 

 the zooecia at different ages. The old zooecia often become 

 entirely altered in form, by the deposition of additional calcareous 

 matter, or by the loss of certain parts present in the younger 

 zooecia, Thus the marginal spines may be entirely lost in the 



Fig. 255. Illustrating the nature of a secondary orifice (Cheilostomata). A, Mucro- 

 nella coccinea Abildg., Scilly Is., x 40. The ovicell (o) overhangs the primary 

 orifice, which is concealed by the great development of the peristome, pro- 

 duced into the mucro (mu) ; t, the three teeth (denticles) within the secondary 

 orifice ; a, avicularium. B, Porella compressa Sowb., Norway, x 40 ; p.o. primary 

 orifice, above which is a concave lamina, the beginning of the ovicell. In the lower 

 zooecium the ovicell (o) is further grown. The primary orifice is still visible, but 

 it is partially concealed by the growth of the peristome, which encloses a minute 

 avicularium ; on, mandible of avicularium. C, Older part of the same colony ; pr, 

 peristome ; s.o, secondary orifice ; o\ adult ovicell ; p, pores. 



older individuals, while in those forms which develop a " peri- 

 stome" (see Eig. 255 and p. 524), the characters of the orifice 

 can often be determined in the young zooecia only. It is 

 thus essential to examine the growing ends of the branches or 

 the rim of the colony, as the case may be. 



In order to make preparations with the tentacles expanded, 

 hydrochlorate of cocaine, chloral hydrate or spirit should be 

 added gradually to the water. When the animals are completely 

 anaesthetised they may be killed by means of a 7-10 p.c. solution 

 of sulphate of copper (best made in distilled water or in rain 

 water). This method gives admirable results in the case of both 



