164 LOWER INVERTEBRATES. 



not find any free-swimming echinoderm larvse in the southern ocean. In the species 

 which develop directly, some of the plates and spines are modified so as to afford 



protection to the ova and em- 

 bryos, which remain attached 

 to the jjarent during the early 

 stages of growth. 



Most of the regular sea- 

 urchins affect rocky coasts, 

 and many of them, by what 

 agency is not known, burrow 

 into limestone rocks and coral 

 reefs until they lie in a cavity 

 which fits their bodies. 



The total number of genera 

 of Echinoidea known is not 

 more than two hundred and 

 twenty-five, represented by 

 about two thousand fossil, and 

 less than three hundred recent 

 species. 



Fl(i. 14o. — \tn\i\got Strong ijlocentrotlis, m f 



1 wenty-iour genera of 

 echini now living, including several spatangoid forms, were already existing at the 

 time of the earliest tertiary foi-mations, and some of these date back to the Jurassic 

 beds, or even to the lias and trias. In tertiary times occur thirty-eight additional 

 genera which have come down to the jjresent time. TJie tertiary fossil echinids of the 

 European beds are so similar to those now living in the West Indies, that it is nearly 

 impossible to distinguish the species. 



The southern ocean is the home of most of the deep sea or abyssal species, some 

 fifty in all, and only one of these, Pourtalesia lyhiale., extends into Europe in deep 

 water, though a comparatively large number of Pourtalesice and Echinothurida^ extend 

 into the North Pacific. Twelve of the abyssal species extend beyond two thousand 

 fathoms. Forty-six species may be called continental, occupying an intermediate posi- 

 tion between the littoral species and the abyssal forms. Ten of these sjiccies extend to 

 great depths. 



The orders of the Echinoidea adojited by A. Agassiz in his report ujion the results 

 of the 'Challenger' expedition are the Palsoechinoidea (extinct), the Desmosticha 

 or regular sea-urchins, the Clypeastridm or cake-urchins, and the Petalosticha or 

 irregular sea-urchins. 



Order I. — DESMOSTICHA. 



This order includes those sea-urchins which have a perfectly regular form, the ambu- 

 lacra commencing at the aperture of the mouth and continuing around the test, which 

 is more or less globose, until they reach the apical system in the centre of the upper 

 aspect of the test. The mouth and anus are thus in this order always to be found 

 upon opposite aspects, the ambulacra divide the circle of the test at five equal 

 angles, and, except in a very few instances (in the Echinometridae) there is no dif- 

 ference in length between the two equatorial diameters of the body. 



