ALCYONARIA. 



177 



The polyps (autozopids) have eight pinnate tentacles and eight 

 mesenteries (Fig. 146). The gonidial groove (siphonogiyphe) when 

 present is single, and the side on which it is placed is called ventral. * 

 The longitudinal muscles are placed on the ventral faces of the 

 mesenteries (Fig. 146). The dorsal mesenteries are often longer 

 than the others, and are developed earlier in the bud, though later 

 in the egg. The polyps vary considerably in their power of con- 

 tractility. In the genera, with a good development of spicules in 

 the body-wall, there is hardly any retractility, but the tentacles are 

 simply folded over the mouth on irritation. In Stereosoma, which 

 has a horny layer beneath the ectoderm, the polyps are non-retractile. 

 The polyps are often dimorphic. There are the autozooids with 

 tentacles and generative organs, and 

 the siplwnozooids without these struc- 

 tures, and with filaments only on 

 the dorsal mesenteries. A gonidial 

 groove (siphonogiyphe) is present 

 in the Aleyonidae and in 8c(ir<>/>lit/- 

 ton ; it is absent in the autozooids of 

 Pennatulacea, Heteroxenia, and Para- 

 (/orf/ia, but present in the siphono- 

 zooids, in which it is always specially 

 developed. The Pennatulacea are 

 phosphorescent. 



The enteric cavities of the polyps 

 are connected with the fine canal- 

 system of the coenenchyma, when 

 such exists, and are continued as 



main canals for a longer or shorter distance towards the base of 

 the colony. In the Stolonifera they all, of course, open into the 

 basal stolon. 



Development. The ova develop inside (as far as the planula) and 

 outside the parent. They have yolk, and the segmentation is on the 

 centro-lecithal type, and is often delayed until the nucleus has under- 

 gone many divisions. There is usually a free-swimming planula-larva. 



Extinct forms.f The genera Heliolites (palaeozoic) and Poly- 

 tremacis (chalk, greensand, eocene) were probably Helioporidae, and 



* This is a special use of the term ventral, and does not imply any homology 

 with the ventral surface of bilateral animals. 



t K. A. Zittel, Handbuch der Palacontologie, Bd. 1, p. 208. Munich and 

 Leipzig, 1876-80. 



N 



FIG. 146. Transverse section through 

 Alcyonium (after Hertwig). R goni- 

 dial groove ; 1, 2, 3, U, the four pairs 

 of septa with their muscles. 



