468 THE ANATOMY OF INVERTEBRATED ANIMALS. 



greatly elongated and cylindrical, the mouth being placed at 

 one end and the anus at the other. The oral aperture is sit- 

 uated in the centre of a circle of tentacula, and the gullet 

 leads from it to an alimentary canal, without marked distinc- 

 tion of stomach and intestine, which extends through the 

 body, and is connected by a mesentery with the parietes of 

 the latter. The wall of the alimentary canal presents exter- 

 nal circular, and internal longitudinal, muscular fibres, and 

 its cavity is lined by a cellular endoderm. 



The body-wall, or perisoma, consists of an external cellu- 

 lar ectoderm, covering a layer of connective tissue within 

 which are circular and longitudinal muscular fibres. The lat- 

 ter are disposed in five bands, attached anteriorly to a corre- 

 sponding number of the pieces of a calcareous ring which 

 surrounds the gullet (Fig. 136, E). The separate ossicles 

 which compose this ring are usually ten or twelve in number, 

 and the five to which the longitudinal muscles are attached 

 are notched or perforated for the passage of the ambulacral 

 nerves, which proceed from the circum-cesophageal nerve to 

 the parietes of the body. 



The integument contains numerous perforated, flat, calca- 

 reous plates, to which protruding anchor-like hooks of the 

 same substance are attached (Fig. 136, F). According to 

 Semper, these anchor-like bodies are developed in special 

 sacs with an epithelial lining. 1 



A spacious peritoneal cavity lies between the parietes of 

 the body and the alimentary canal, and the cells which line it 

 are more or less extensively ciliated. Pedunculated ciliated 

 cups are attached to the mesentery. 



The circular vessel of the ambulacral system surrounds the 

 gullet below the calcareous ring (Fig. 136, E, h). Posterior- 

 ly, it gives off various csecal prolongations, which depend 

 freely into the peritoneal cavity. Some of these — the Polian 

 vesicles — are mere caeca ; but, in addition, there are one or 

 more tubular prolongations, the perforated extremities of 

 which are invested by a calcareous network, and are termed 

 the madreporic canals. Through the openings in the free 

 end of the madreporic canal, the interior of the ambulacral 

 system communicates with the peritoneal cavity. Anterior- 

 ly, the circular vessel gives off branches to the tentacula. 

 These pass between the calcareous ring on the outer side, 



1 See, on this and all points relating to the structure of the Holoihuridea, 

 the beautiful monograph by Semper, t? Reisen im Archipel der Philippinen." 

 (" Wissenschaftliche Resultate," Bd. i. : HolotJiurien.) 



