170 



LOWER INVERTEBRATES. 



H. mammilatus has ten to eleven pairs of pores in each arc, while IT. trigonarius has 

 fifteen to seventeen pairs. The spines of the former are usually stout and bat-shaped, 

 and in color vary from uniform ash-gray or light brown, with white wings at the end, 

 to nearly black. In H. trigonarius the spines are usually longer, tapering, and more 

 or less triangular, but Agassiz states that they vary so much that the two species 

 cannot be distinguished by the spines. When a spine of H. mammilatus is broken 

 off at the base, it is replaced by a long tapering triangular spine like that of the other 

 species. 



The genus to which the cumbrous name of Strongylocentrotus has been given con- 

 tains species with a circular or pentagonal, slightly depressed test, with pores arranged 

 in arcs of at least four or five pairs. There are several species, of which the best 



known is the S. drobachiensis of the 

 "'^'^'tA-' ' north-eastern coast of North America, 



and of Alaska. S. mexicanus occurs 

 in the Gulf of California, and the test 

 reaches a diameter of nearly three 

 inches ; but these dimensions are far 

 behind those of S. franciscanus, of 

 the west coast of the United States. 

 In this foi-m the test alone is five or 

 six inches across, and the spines are 

 large, so that fine examples measure a 

 foot. In the Echinidffi proper, a group 

 which contains several genera and 

 species, the test is often nearly globu- 

 lar, — in AmUypneustes the height 

 equals the width. Echinus esculentus 

 is one of the best known forms, and is found on the coasts of Noi-way and of England. 

 The test is of a brownish or brick red color. As its name implies, it is occasionally 

 used as food. Ilippono'e depressa is a large species from the western coast of Mexico 

 and the Gulf of California. 



Pri07iechinus sagittiger, a species found by the ' Challenger' expedition at depths 

 varying from seven hundred to one thousand and seventy fathoms in the seas around 

 Australia and the East Indian islands, is remarkable from the presence upon the spines 

 of serrations resembling those of Salenia varispina, instead of the regular fluting 

 characteristic of most Echinidse. 



Fig. 149. 



'Echinus esculentus, the spines removed from 

 half of the test. 



Okdek II. — CLYPEASTRIDiE. 



In this order the mouth is placed as in the regular sea-urchins, but the anal open- 

 ing occupies a position immediately opposite to the odd ambulacrum, and often on 

 the under side. The genital pores retain their position at the summit of the upper 

 surface of the test, which is exceedingly depressed, with its edge or ambitus more or 

 less sharp, so that the upper and under surfaces are entirely cut off from each other, 

 and are of quite different character. The rows of pores for the exit of the suckers 

 do not extend around this sharp edge, but form five pairs of curves, arranged some- 

 what like the petals of a flower, upon the upper surface only; while on the oral 

 surface the ambulacra are marked by furrows that converge toward the mouth. The 



