264 AMONG THE SEA URCHIXS. 



were in this specimen {Echinus esaile?itiis or sphccrd)^ as in most 

 of the regular Echini, five pairs of external branchiae or oral gills, 

 which, being very feathery and thin, assist in respiration, for they 

 are ciliated internally and communicate with the water vascular 

 system within the test, which is thus kept oxygenised. No young 

 naturalist ought to miss the interesting sight which is gained by 

 placing the mouth portion of a small echinus under the micro- 

 scope with a good top light, as he will thus obtain more practical 

 knowledge of the wonderful construction of this walking ball than 

 by any drawings or descriptions, however lucid. 



Referring again to the woodcut. Fig. C, on p. 15, the teeth, 

 which are quite soft at the top and bend over, are seen going down 

 the centre of each pyramid. At Fig. A are seen the plates of the 

 summit of the masticatory apparatus, d, and some of the striated 

 muscles, e, while above each "brace" dividing the pyramids are 

 long bifid processes, called the " compass-pieces " or " radii." 

 Each brace radiates from the central axis of the jaws and joins in 

 its lower portion the curious " perignathic girdle," formed inside 

 the test in alternate ridges and arches, beneath which latter the 

 radiating, ambulacral canals, shown on p. 15 at ^^ in Fig. 2, C, 

 have, as we have said, sufficient space allotted them. 



The teeth move in a circle around the opening of the gullet, 

 which passes upwards through the " lantern," and is continued 

 into an elongated alimentary canal, shown at a, in Fig. A, on 

 page 15. This intestine winds from left to right, being coiled 

 spirally around the interior of the shell, and is kept in position by 

 mesenteric bands and filaments, terminating at the " periproct," or 

 anaPaperture, Fig. C, a, on page 11. 



The partially digested food found in the stomach consists 

 of pellets^ of algae, etc., and this confirms our idea that the 

 Echini were feeding upon the sea-weed when we watched their 

 proceedings so closely in the deep cavern to the south of Port 

 Erin Bay. 



The surface of the mesenteries, as well as the lining membrane 

 inside the shell, is thickly ciliated, and this, no doubt, is helpful 

 in circulating the richly amaeboid fluid, filling the test, as well as 

 assisting in respiration. The regular Urchins possess two 



principal blood vessels, one round each end of the alimentary 



