THE CRUSTACEANS 111 



are especially abundant upon the beaches at the times of the spring 

 tides. The eggs hatch in about a month, and the young bear a re- 

 markable resemblance to the trilobites so that they are said to be 

 in the trilobite stage. The sharp terminal spine develops later, 

 after moulting. 



Horseshoe crabs have been used for making fertilizer in Dela- 

 ware Bay and are nearly exterminated in that region. They are 

 among the most interesting of all marine animals, and are probably 

 the last survivors of a race which may have been a connecting link 

 between the trilobites, crustaceans, and scorpions. There are very 

 few existing species, one being found on our coast and several on 

 the coast of Asia. 



THE SEA SPIDERS 



Pycnogonidce. 



These are generally small creatiires Avhich crawl slowly over 

 hydroids and sea weeds, and bear a superficial resemblance to 

 sj)iders. Their true relationships are, however, a puzzle to 

 naturalists. 



The body is extremely small, so that the stomach and re- 

 productive organs extend outward into the long, stout legs, the 

 stomach in some species reaching even to the tips, while the repro- 

 ductive organs open on the lower sides of the second joints. 



There are typically seven pairs of appendages in the male and 

 six in the female, the third pair being absent in the females of 

 many species. In the male, however, these are developed into egg- 

 carryiiig organs, for he gathers the eggs into balls as soon as they 

 are laid, and fastens them to his third pair of legs by means of a 

 cement which issues from openings at the fourth joints of his walk- 

 ing legs. When the young hatch they often crawl over the male 

 in considerable numbers. In other genera the young enter the 

 digestive cavities of hydroids, and form gall-like enlargements upon 

 the stems, within Avhich they live. 



PhoxicMlidium maxillare is a small species which varies in 

 color from blackish-brown through sepia to almost white, and 

 spreads about one inch. It is commonlj'- found crawling over 

 hydroids and sea-squirts, on shelly bottoms. The young live for a 



