112 



INVERTEBRATE MORPHOLOGY. 



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 Zoantheae. 



The Zoanthege form very frequently colonial aggregates 

 either of a diffuse stoloniferons character (Zoanthus] or of a 

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

 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 antl fora- 



ininii'eran shells, etc., imbedded 

 in the outer portion of the rneso- 

 gloea. 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- 



10 j - 



entenes are arranged in pairs, 

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

 ///and IV) correspond with the 

 six primary pairs of the Protac- 

 tinise ; of these the dorsal direc- 

 FIG. 64. DIAGRAMMATIC TUANS- tives (D) are never united to the 

 VERSE SECTION OP ZonntJius. stornatodaeum and the dorsal 

 D D' = directive mesenteries. ^^ [r (// y ^ consigts of 

 I- VI as in preceding figure. 



1-4 = pairs of secondary mesen- ono perfect and one imperfect 

 teries. 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 t) 

 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. 



