ACTINOZOA. 



169 



or some multiple of six ; but it may be different : indeed, the greatest 

 variety is found in this character in the order Zoantharia. 



The gonads are produced on the mesenteries (Fig. 91), aVd the 

 embryos sometimes undergo the early stages of development within 

 the parent. 



Asexual reproduction by budding and fission is of great importance. 

 Buds can be formed in various positions, even at the oral end, in 

 which case a strobila-like form appears. In Blastotrochus the buds 

 appear at right angles to the axis of the parent (Fig. 139). 



In Gonadinia prolifera the polyp divides transversely, a new set 

 of tentacles arising on the lower half (Fig. 139a). In some cases 

 a portion of the basal expansion is separated off by contraction of the 

 body, and develops into a new polyp. This is called laceration. 



FIG. 139. Blastotrochus 

 nutrix (after C. Sem- 

 per). LK lateral bud. 



FIG. 139a. Two stages of transverse fission of Gonactinia 

 prolifera, Sars (after Blochman and Hilger). 



If the individuals so produced remain connected with one another, 

 a polyp-colony is formed, which may attain very various forms and 

 great size. As a rule the individuals are embedded in a common 

 body mass, the coenenchyme* and their gastric cavities communicate 

 more or less directly, so that the juices acquired by the individual 

 polyps penetrate through the whole stock. This stock affords us 

 an excellent example of an animal community built up out of similar 

 members. The formation of the generative products alone is some- 

 times confined to special polyps, which, however, discharge all 

 other functions of polyp life. 



The skeletal formations of the polyps are specially noteworthy. 

 In almost every case, with the exception of Actinia, there is a deposit 

 of solid calcareous matter, and according to the density of this deposit, 

 there is produced a leathery, chalky, or even stony framework. 



* This word is used in a different sense in the Madreporaria, which see. 



