It^O LOWER INVEHTEHRATES. 



North Atlantic. Its j)lace in tlie Xorth Pacific is taken by S. decemradiata. The retic- 

 ulation of the skeleton is close, and the arms less tiattened than in the next species. 



Crossuster papposus is common to hoth sides of the Atlantic, and is found in Nor- 

 way, Denmark, Britain, France, and in America as far south as Massachusetts Ba^•. 

 It has twelve arms, the spaces between which are largely filled up upon the oral side 

 by a membrane, while the u]ij)er surface is an open network of limestone rods, carrying, 

 at the points of junction, club-shaped ])rocesses which bear tufts of small spines. 



Cribrdia sex-radiata, from the ^Vntilles, is remarkable from its possession of six 

 arms, an exceptional character in this genus. It has also the faculty of reproduction 

 by division into two halves, so that most examples show three larger and three smaller 

 arms. This jiower is also possessed liy several of the manj-armed Asterias, by some 

 JJnckias, and some Asterhias. C'rlhvella suuf/iii/ioleiita is common on the New 

 England coast below low-water mark. 



The AsTERiD.E are star-fishes which usually have the arms well-developed, and have 



four rows of water-feet, each ending m a 

 sucking-disc, along the ambulacra. One 

 of the largest genera is Asterias, or Aster- 

 acaiithion, species of which are found 

 ^j&^ everywhere in the northern hemis]ihere. 

 ■^'^'W'- -^- ruhens is a common European form. 

 >j?%?i.'?»S'^~ - 1- beryliims extends from Halifax, Nova 



^Pffiv' - '"''M-i^ Scotia, to Florida, while A. vuh/aris 



V'"" ranges from Long Island Sound to Lab- 



*"' rador. The last two species are both 



~^i common in Massachusetts Bay. 



>^^lis» \'i<?2s?& In A. ochracea, which rantjes from 



/?N«--^V V-'ir,'^^ Sitka to San Dieso, and is the most com- 



/~$^=%f H'h4^\ mon star-fish of the Californian coast, the 



/~*V>' Ikv*^ arms and the ambulacra are wider than 



^o>' X&f^ in most species of the genus, and the cal- 



W careous network which covers in the sides 



and back of the arms is exceednigly solid. 



Several other sjiecies of Asterias occur ujion the Pacific coast of the United States, 



but the most conspicuous is the large six-armed A. <ji(jantea, which attains a diameter 



of more than two feet. Nearly allied is Pycno'podia helianthoides, a gigantic form 



with more than twenty arms, common on the Pacific coast of North America, from 



Cape Mendocino to Alaska; the calcareous skeleton of the upper surface is reduced to 



a few small rods at the base of the sjiines, and hence a large well-preserved specimen 



is a rarity. This species attains the diameter of three feet, or thereabouts, and is of 



a bright red color in life. Professor A. Agassiz considers that this sjiecies, as well as 



Crossaster papjjosus, are in many respects allied to Urisi/if/n. 



The many-armed Asterida? are, for the most part, included in the genus Helittster, 

 or sun-star, two species of which, H. kifbiniji and JT. microbrac/da, occur upon the 

 west coast of North America, from Panama to Cape St. Lucas. The latter form has 

 more than thirty arms, and the free portions of the arms are very short. 



Zoroaster fulgens, dredged northwest of the Hebrides, has immensely long arms 

 and a very small disc, not one-twelfth of the total diameter of the animal, which 

 measures ten inches across. It closely resembles Ophidiaster, but has four rows of 



