504 INVERTEBEATA chap. 



that these flaps have been precociously developed. In dealing with 

 the development of Gastropoda, it will be remembered we saw reason 

 to believe that, in the veliger stage, we had an example of the 

 opposite phenomenon, viz. the retention of a larval organ long 

 beyond the stage when, according to ancestral history, it ought to 

 have disappeared. 



ECHINOIDEA 



The complete life -cycle has been described in a very few 

 Echinoidea. We possess records of the external appearance of the " 

 larva in all stages of development in the case of Echinus miliaris 

 and Echinus esculentus, two species of regular Endocyclic urchins ; 

 also in the oases of Echinocyamus pusillus and Mellita testudinata, two 

 Clypeastroids ; and of Echinocardium cordatum, a Spatangoid. In 

 the case of Echinus esculentus the development has been thoroughly 

 investigated by us by means of sections (MacBride, 1903), and we 

 shall select this species as a type for detailed description. 



Owing to the fact that the ovaries and testes in Echinoidea are 

 larger in proportion to the body and more compact than in any 

 other class of Eehinodermata, a very large proportion of the ex- 

 periments, on which the science of " Experimental Embryology " is 

 founded, have been performed on the eggs of Echinoidea. With 

 hardly an exception, however, the experimenters have confined their 

 attention to that part of the development which terminates in the 

 formation of the four-armed larva ; and further, the overwhelming 

 majority of the experiments have been performed on the three 

 species most commonly found at Naples, viz. Echinus microtuberculatus, 

 Strongylocentrotus lividus, and Sphaerechinus granulans. All three 

 are regular urchins, and, in all important points, all three have given 

 identical results in the hands of experimenters. 



When we reflect that Echinus Tnicrotuberculatus is only a local 

 race of Echinus miliaris, and that, except for insignificant details, the 

 early development of Echinus miliaris and Echinus esculentus is the 

 same, and that the development of Strongylocentrotus lividus, which 

 has recently been investigated by von tjbich (1913), seems to be in 

 all respects similar to that of E. miliaris, we shall, without hesitation, 

 apply to Echinus esculentus the minute knowledge of the earliest 

 stages of development which has been gained by the concentration 

 of the attention of the experimenters on the Neapolitan species. 



We shall first describe the normal development and then briefly 

 sketch the principal results arrived at by experiment. 



The eggs of Echinus esculentus can be made to develop through 

 their complete life-cycle even when artificially shaken out of the 

 ovary. It is by no means necessary to wait for a natural spawning 

 in order to get good results. Echinus esculentus is a gregarious 

 form and can usually be obtained in large numbers, either by 

 dredging at moderate depths, as in Plymouth Sound, or by picking 

 it off the rocks at low tide in more sheltered'situations, as in the Clyde. 



