480 COMPARATIVE ANATOMY chap. 



the anus, out of the apical system into the posterior interradius. The 

 mouth therefore draws the commencement of the first intestinal coil 

 (which runs in the direction of the hands of a clock) forward, while 

 the anus draws back {i.e. posteriorly) the end of the spiral which runs, 

 as above described, backwards (Fig. 382, F, p. 475). 



It is worthy of note that, in quite young Spatangoidea (Hemiaster 

 caveriwsus, 2 mm. long), the intestine, which appears to end blindly, 

 ascends direct from the oral to the apical pole. At a rather later 

 stage the mouth is still central, while the apical end of the alimentary 

 canal has already somewhat shifted, and opens through the anus out- 

 side of the apical system. At this stage (when the length of the 

 animal is from two to three mm.) the intestine runs up from mouth 

 to anus in one single coil, as a spiral in the direction of the hands 

 of a clock. The complicated arrangement in the adult is thus 

 secondary, and is no doubt due to the fact that the canal increases in 

 length more than does the interval between mouth and anus. 



Finer structure of the intestinal wall. — This agrees essentially witli the struc- 

 ture <lescribed in couneetidn witli tlie Holotliurioidea. No distinct sections can be 

 made out in the alimentary canal. That }iart of it which runs through the mastica- 

 tory apparatus is often called the pharynx. Its lumen in section is five-rayed, the 

 layer of connective tissue forming five longitudinal ridges which bulge in the 

 epithelium. It is connected, in a manner which cannot here he further described, 

 bv means of five pairs of longitudinal bands of connective tissue, with the surround- 

 ing masticatory apparatus. 



The name oesophagus is generally given to the portion of the digestive tract 

 which follows the phai-ynx (and, in the Spatangoidea , to the wliole of tlie first por- 

 tion of the intestine) as far as the point where, in regular Echinoids, there is a sac- 

 like widening, and in the Spatangoidca a large ciecum. In regular Echinoidea, it 

 includes that part of the intestine which ascends from the lantern towards the 

 apical system, together with the first portion of the first spiral. In the Spatangoidca 

 it runs back from the mouth and then bends forward, forming the commencement 

 of the first intestinal spiral. 



The a-sophagus is followed by the first intestinal spiral, which runs in the 

 direction of the hands of a clock. It commences with a slight sac-like swelling 

 (regular Echinoids) or a large crecum {Spatangoidea.). In this part of the alimen- 

 tary canal a rich system of blood lacunre is developed in the connective tissue layer 

 on the inner side of the otherwise weak musculature. 



In the second or reverse spiral this lacunar network is wanting. This spiral is 

 distinguished, more especially in the regular Echiuoidea, by its peculiar colouring, 

 being yellow, whereas the first spiral appears brown. 



In regular Echinoidea, the two intestinal spirals have an elegantly undulating 

 course, regularly ascending and descending. 



The second spiral passes without any sharp boundary into the rectum, which, in 

 the Spatangoidea, runs back from the middle of the body. At its commencement, in 

 Echinocardium {flavcsccns) and Schizaster, it has a small diverticulum. 



The alimentary canal of the Spatangoidea, which is distended with sand and 

 mud, is thicker and its walls are firmer than in the regular Echinoidea, whose in- 

 testine usually contains, besides mud, a large number of unicellular algte. There is a 

 corresponding ditference in the mesenteries. In the regular Echinoidea the intes- 

 tine is attached by means of mesenteries practically only to the test, and these 



