158 LOWER INVERTEBRATES. 



crowned with small tuftud spines called paxilli. They are most numerous in the hot 

 seas. 



The genus Astropectcii is a large one, and many species occur in deep water. A. 

 articulatus ranges from New Jersey to the West Indies. Several species are known 

 to occur on the west coast of North America. 



Nearly allied to Astropecten, but differing from it in the very conspicuous feature 

 of the absence of any large marginal plates on the sides of the rays, is I^uidia, the 

 brittle star-fish, celebrated for its j)ower of breaking into fragments when it is brought 

 from the bottom. L. cluthrata ranges from New Jersey to the West Indies, and is 

 common in the Carolinas. Z,. tessellata, a fine species, attaining a diameter of more 

 than a foot, ranges from the Gulf of California to Peru. Ctenodiscus crispatus is 

 another North Atlantic species, occurring in the North Sea, Greenland, and New Eng- 

 land. It is almost pentagonal from the shortness of the rays. 



Lepty chaster kerguelenensis has its dorsal suiiace covered with a tessellated jiave- 

 ment of paxilli or spines, with large heads. These paxilli form with their approxi- 

 mated heads a sort of hexagonal mosaic on the surface, but between their slender 

 shafts arcade-like spaces are left, and into these spaces the eggs pass from the genital 

 openings. Eggs and young in all the early stages of development occupy these spaces 

 at once, but when at least six suckers have formed on each arm, they push their way 

 out between the paxilli, usually in the angles between the arms. The young escape 

 mouth uppermost, and disengage their arms one by one. After this they remain at- 

 tached to the parent for some time. In the young, the madreporic plate can be seen 

 near the margin of the disc, but in the adult it is hidden by the paxilli. 



Archaster vexillifer, taken in tliree hundred and forty-four fathoms, west of the 

 Shetland Isles, is a fine sj)ecies about ten inches across, remarkable for the arrange- 

 ment of the ambulacral spines, which form combs that increase in size toward the 

 base of the arms. Each plate has a double row of spines, and each spine has a 

 second short spine at its end. The ambulacral grooves are much wider, and the 

 ambulacral tubes larger in proportion to the animal than is usual. 



Archaster bifrons, taken in deep water north of the Hebrides, measures five inches 

 across, and is of a rich cream or light rose tint. 



Porcellanaster cceruleus w;is dredged by the 'Challenger' in one thousand two 

 hundred and forty fathoms, in the Gulf Stream. The arms are rather shorter than the 

 diameter of the disc ; the ambulacral plates are large, and furnished with two flat- 

 tened spines ; and the plates forming the angles of the mouth are unusually flattened 

 and expanded. There are two rows of large marginal plates, and the two on the end 

 of each arm are fused together, and bear three spines, one on each side below, and a 

 central one above. Vertical rows of small flattened scales cover the two central 

 pairs of marginal plates in the re-entering angles between the arms, and look like a 

 little brush between each pair of ai-ms. The upper surface is covered with narrow 

 calcareous plates, and is of a delicate cobalt blue color. The excretory opening is 

 very distinct in the centre of the disc. It has been found in the North Pacific and 

 near Tristan d'Acunha. 



The AsTERiNiD^ are pentagonal, more or less elevated in the centre of the disk, 

 and always sharp at the edge. Spines are present, at least on the ventral surface, and 

 there are two rows of pedicels in the ambulacral grooves. 



Asteropsis imbricata, a species which ranges from Vancouver's Island to near San 

 Francisco, is a good example of this family. The skeleton of the aboral side is a retic- 



