110 



INVERTEBRATE MORPHOLOGY. 



the arraugement of the retractor muscles as shown in Figure 

 57. 



3. Order Cerianthese. 



The Cerianthese are, like the Edwardsise, solitary forms 

 destitute of a skeleton, and live imbedded in sand or mud. 

 The basal region is rounded and not adhesive, having at the 

 centre a pore which communicates with the ccelenteron. In 

 Cerianthus a fibrous investment surrounds the body as a tube, 

 secreted by the ectoderm, this layer of the body being further 

 characterized by an enormous development of muscle-fibres 

 arranged longitudinally and supported upon slender processes 

 of the mesoglcea of the body-wall. The tentacles are simple 

 and very numerous, being arranged in two sets, one surround- 

 ing the margin of the funnel- 

 shaped disk and the other im- 

 mediately surrounding the 

 mouth. The mesenteries are 

 also very numerous and are 

 distinguished by the absence in 

 the adult of retractor muscles, 

 the ectodermal muscles playing 

 the part of the retractors, and 

 the characteristic Anthozoan 

 sphincter is also absent. The 

 arrangement of the mesenteries 

 (Fig. 62) is peculiar to the 

 group, new ones continuing to 

 form during the entire life of 

 the animal and making their 

 appearance one on each side 

 of the sagittal plane between 

 the two which immediately preceded them. The older mes- 

 enteries are thus crowded to one surface of the body, the 

 dorsal surface, at which the single siphonoglyphe (si) occurs 

 in the stomatodseum, and the four on either side of the 

 mid-dorsal line (I-IV) are the homologues of the eight 

 mesenteries of the Edwardsiss, the rest being secondary 

 structures not represented in that group. 



TlGt. 62. — DlAOBAlOtATIC Trans- 



VERBK Section op a Young Ceri- 

 anthus (according to Carlgkbn). 

 si = sipbonoelyphe. 

 I-IV = the Edwardsian mesenteries. 

 1-4 = secondary mesenteries. 



