112 



INVERTEBRATE M0RPHOL0O7. 



In all these forms there is a strictly bilateral arrangement 

 of the mesenteries, and a tendency for them to arrange them- 

 selves in pairs. 



6. Order Zoanthese. 



The Zoanthese form very frequently colonial aggregates 

 either of a diffuse stoloniferous character (Zoanthus) or of a 

 more compact form, the individuals being imbedded in a coe- 

 nenchyme (Palythoa). No skeleton is present, though many 

 forms have a dense crust on the outside of the body formed 

 of particles of sand, sponge-spicules, radiolarian and fora- 



miniferan shells, etc., imbedded 

 in the outer portion of the meso- 

 glcea. They possess a varying 

 number of simple tentacles, and 

 there is only a single siphono- 

 glyphe which marks the ventral 

 surface of the body. The mes- 

 enteries are arranged in pairs, 

 six of which (Fig. 64, /- V, II- VI, 

 III and IV) correspond with the 

 six. primary pairs of the Protac- 

 tinise ; of these the dorsal direc- 

 tives (Z>) are never united to the 

 stomatodasum and the dorsal 

 lateral pair (//, VI) consists of 

 one pei'fect and one imperfect 

 mesentery, the latter being ventral 

 to the former. The ventral lat- 

 eral primary pair may consist of two perfect mesenteries or 

 may have the same arrangement as the dorsal lateral pair. 

 To these six pairs a varying number of secondary pairs (1-4) 

 may be added, the new pair always arising immediately on 

 either side of the ventral directives. Each of the new pairs 

 consists of a perfect and an imperfect mesentery, the latter 

 being the dorsal one of the two, these secondary pairs thus 

 differing from the lateral primary mesenteries. 



Fig. 64. — Diagrammatic TranS' 



VERSE Section of Zoanthus. 

 D, D' = directive mesenteries. 

 I-VIss, in preceding figure. 

 1-4 = pairs of sec6ndary mesen 

 teries. 



