164 



LOWER INVERTEBRATES.. 



not find any free-swimming echinoderm larvae 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 parent during the early 

 stages of gi'owth. 



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, rejDresented by 

 about two thousand fossil, and 

 less than three hundred recent 

 species. 



Twenty-four genera of 

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

 time of the earliest tertiary formations, 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 present time. The 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 2?hiale, extends into Europe in deep 

 water, though a comparatively large number of Ponrtalesice and Echinothuridse extend 

 into the North Pacific. Twelve of the abyssal species extend beyond t\yo 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 species extend to 

 great depths. 



The orders of the Echinoidea adopted by A. Agassiz in his report upon the results 

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

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

 irregular sea-urchins. 



Fig. 143. — Toimg of Strongylocentrotus. 



Oedee I. — desmosticha. 



This order includes those sea-urchins which have a pei-f ectly 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 tesj; at five equal 

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

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



