184 



COELENTERATA. 



The Zoantheae alone are colonial. As a rule there is a single corona of 

 tentacles at the edge of the oral disc ; these are the primary (marginal) 

 tentacles. When they are in several rows the inner are the oldest. They arise 

 both from intra- and intermesenterial spaces (see below, p. 184). In addition to 

 these there are in some forms (Corallimorphidae) secondary or accessory tentacles 

 arising from the disc midway between the inouth and margin, and these are 

 always intramesenterial. There is nearly always a sphincter muscle at the 

 peristomial margin, which may be endodermal or mesodermal (i.e. in the jelly). 

 It closes the peristomial margin ove*r the mouth and tentacles during retraction. 

 The mouth is usually slit-like, and the oesophagus has two gonidial grooves 

 (Gosse). Sometimes there is only one gonidial groove (Peachia, Zoantheae, 

 Cerianthus). The thickened walls of these grooves are prolonged beyond the 

 lip at the two ends of the long axis of the mouth as two tubercles (called in 

 the Siphonactinidae the conchula). 



The oesophageal lappets are processes of the oesophageal wall at each end of 



the long axis which hang down 

 into the coelenteron beyond the 

 rest of the oesophagus ; the gonidial 

 grooves are continued on to them. 

 The gonidial grooves are, in this 

 work, denned as dorsal and ven- 

 tral. The ventral groove is the 

 most conspicuous, and the only 

 one present in Peachia and the 

 Zoantheae. When there is only 

 one groove, it is not always pos- 

 sible to determine whether it is 

 dorsal or ventral. In the termi- 

 nology which has more recently 

 been introduced by Haddon the 

 ventral groove is termed the sulcus, 

 and the dorsal the sulculus. It 

 will, of course, be understood that 

 the above use of the words dorsal 

 and ventral is special to the group, 

 and implies no homologies with 

 the dorsal and ventral surfaces of 

 other animals. 



The mesenterial arrangement 

 presents the greatest variation. 



The arrangement* generally described as typical is that of the Hexactiniae, in 

 which the mesenteries are arranged in pairs (Fig. 149), and in the simplest cases 

 in six pairs. These are the primary mesenteries, and they all reach and are 

 inserted into the oesophagus. The mesenteries of each pair are usually provided 

 with longitudinal muscles on those faces which are turned towards one another, 

 except on the two pairs of directive mesenteries ; these carry the longitudinal 

 muscles on the faces turned from one another. The portion of coelenteron enclosed 

 in each pair of mesenteries is called an intramesenterial space or endocoele, the 

 portion between the pairs being intermesenterial (exocoele). There are, there- 

 fore, six of the former and six of the latter. There are in many cases secondary 



* For variations see descriptions of the different sections. 



FIG. 149. Transverse section through Adamsia 

 (Sagartidae) (after R. Hertwig). Hf the cham- 

 bers between the directive mesenteries ; R 

 gonidial grooves. 



