354 ZOOLOGY SECT. 



(Fig. 281, int. ccc-c), each with several short branches of irregular 



shape. 



Ambulacral System. Running downwards from the madre- 



porite to near the border of the mouth is an S-shaped cylinder,. 



the madreporic or stone-canal 

 (Figs. 278, m, 284, mad. can), 

 enclosed in a membranous sac. 1 

 The walls of this canal are sup- 

 ported by a series of calcareous 

 rings, projecting from which 

 inwards is a ridge which bifur- 

 cates to form two spirally rolled 



x xJ5 a^WP'SW! iSiSy^ lamellae occupying a consider- 

 Xv^; able part of the lumen of the 



canal. In some Starfishes, such 



lie,. 282. Transverse section through the . / _. rt o\ ,-, 



inadreporic-canal of a Starfish (Axt.-r,- US Astl'Opecten (V iff. z8z) the 



reuscher.) in t. emal structure is more com- 

 plicated owing to the branching 



of the lamellae. The interior of the madreporic canal communicates 

 above with the exterior through the grooves of the madreporite. 

 At the bottom of each of the grooves is a row of pores leading into 

 a sac, the ampulla, which in turn leads to the madreporic canal. 

 Below, the latter opens into a wide, five-sided, ring-like canal, the 

 ring-vessel of the ambulacral system. From this are given off the five 

 radial ambulacral vessels, passing to the extremities of the arms. 

 From the pentagonal canal are given off also a series of five pairs of 

 appendages, the Polian vesicles (Fig. 278, ap ; Fig. 283, pol. ves) 

 pear-shaped, thin-walled bladders with long narrow necks which 

 are placed inter-radially. On the sides of the neck of each Polian 

 vesicle (except in the inter-radius containing the madreporic 

 canal, where there is one on one side only) project inwards a pair 

 of little rounded glandular bodies, the racemose vesicles, or Tiede- 

 manns vesicles, the interior of each of which is divided into a 

 number of chambers. 



The various parts of the ambulacral system of vessels have a 

 muscular wall and an internal lining epithelium, in addition to the 

 coverings which they may derive, according to their situation, 

 either from the external epidermis or the internal coelomic epi- 

 thelium. The muscular layer is most strongly developed on the 

 tube-feet, where it consists of two strata, and is also well developed 

 on the ampulla? and Polian vesicles. 



Accompanying the madreporic canal there is an organ the 

 ovoid gland the relationships and function of which have given 

 rise to a considerable amount of difference of opinion. It is a 

 fusiform body, the interior of which is divided up into a number 

 of freely-communicating spaces. In the interior of these spaces 



1 The so-called "pericardium." 



