162 PHYLUM ECHINODERMATA. 



never becomes separated by mesodermal tissues from the tract 

 of ectoderm which gave it origin in the embryo. This is a 

 feature of all Echinoderms in so far as the ventral nervous 

 system, which is the predominant central nervous system, is 

 concerned. When this nervous system is removed from the sur- 

 face, the removal is effected by invagination (p. 124). 



(2) The presence of calcined skeletal tissue in the meso- 

 derm of the body wall is a character found in Echinodermata 

 and Vertebrata alone among Coelomata. This has already 

 been pointed out by MacBride, and though not perhaps a very 

 important indication of affinity is one which from its rarity 

 deserves mention here. 



(3) In all Echinoderms the mouth shifts from the ventral 

 surface of the larva on to the left side of the body. This can be 

 demonstrated in all classes except Crinoids, and in Crinoids it 

 may fairly be inferred. In Chordates a similar though not 

 identical phenomenon is presented by > Amphioxus. In this 

 animal the mouth actually makes its appearance on the left 

 side in an animal otherwise bilaterally symmetrical, but the 

 phenomenon differs from that of Echinoderms in the fact that 

 the left-sided position of the mouth is not preceded by a con- 

 dition in which it is in the middle ventral line. The feature 

 then which Echinoderms have in common with Amphioxus is 

 the sinistral position of the mouth. Here again we have a 

 character which strikes us from its very rarity, for it is found 

 in no other Coelomate nor so far as we know in any other mem- 

 ber of the animal kingdom. It also strikes us by its strangeness 

 and inexplicableness. In Amphioxus no serious attempt has 

 been made to explain it. 



In attempting to explain peculiarities of this kind we are 

 accustomed to take into consideration two factors which must 

 be kept distinct. Firstly we have peculiarities in habit, secondly 

 associated peculiarities in other organs. Now in Amphioxus the 

 asymmetry of the mouth is accompanied by no peculiarity in 

 habit, for the animal while it has this monstrous mouth behaves 

 more after the fashion of a bilaterally symmetrical animal than 

 it does in later life, when the mouth has acquired a more median 

 position and it has taken to burrowing in sand. Nor do the 

 peculiarities in some of the other organs lend us any assistance, 

 for no one, so far as we know, has ever attempted to bring the 



