ALIMENTARY CANAL OF ECHLNODERMA. 



213 



form several rows one above the other (Fig-. 102, d). In these 

 therefore the dermal skeleton forms the organs for the comminution 

 of the food. A short, wide oesophagus follows the mouth ; this is 

 continued into a wide mid-gut (stomach), which occupies the middle 

 of the body. In the Ophiurida and many Asterida (Astropecten, 



Fig. 107. Transverse section through the arm and disc of Solaster en dec a. 

 The radial and the interradial portions are figured on opposite sides. o Mouth. 

 v Stomachal cavity, c Radial ceeca. g Genital gland, m Madreporic plate, s Stone- 

 canal with its so-called heart, p Ambulacra! feet (after G. 0. Sars). 



Luidia) the stomach is always a blind sac, as it is also in Brisiuga. 

 But in all Astero'i'da it is provided with diverticula or cascal 

 saccular appendages, which are indicated in the Ophiurida by 

 radial constrictions. The gastric casca of the Asterida extend in 

 pairs into the arms ; they 

 spring from the stomach, and 

 have the form of thin-walled 

 tubes, closely beset with 

 lateral appendages (Figs. 

 107, c; 108, h), which as a 

 rule are united by pairs into 

 one canal before they open 

 into the stomach. This tract 

 represents an unpaired por- 

 tion of the enteron belonging 

 to each antimere (arm) of the 

 Astero'i'da, while the cascal 

 tubes form a paired por- 

 tion. In Astropecten auran- 

 tiacus these tubes arise sepa- 

 rately from the stomach. The 

 unpaired portion in each arm 

 has therefore disappeared in 

 this form, and with it the 

 primitive condition. In most 

 of the Asterida the short hind-gut is continued from the stomach 

 to the anus, which is placed on the dorsal surface. 



The enteric tube of the Crino'i'da (Comatula) is modified; it describes 

 a spiral coil, and its narrower short terminal portion passes into a 

 tubular and projecting anus, which is placed interradially near the 

 mouth. This coiled arrangement, which is apparently very anomalous 



Fig. 108. Asteriscus verrnculatus: 

 opened on the dorsal surface. a Anus. 

 i Rosette-shaped enlarged enteron (stomach). 

 h Tubular radial appendages of the enteron. 

 g Genital glands. 



